[protocol] Remove unused GETHEADERS_VERSION
[protocol] Remove unused GETHEADERS_VERSION

GETHEADERS_VERSION is unused. No need to keep it in the codebase
for just its historic comment.

JeremyRubin/bitcoin,instagibbs/bitcoin,qtumproject/qtum,MarcoFalke/bitcoin,fanquake/bitcoin,mm-s/bitcoin,bitcoinsSG/bitcoin,Xekyo/bitcoin,cdecker/bitcoin,kallewoof/bitcoin,andreaskern/bitcoin,particl/particl-core,sipsorcery/bitcoin,JeremyRubin/bitcoin,instagibbs/bitcoin,practicalswift/bitcoin,JeremyRubin/bitcoin,qtumproject/qtum,mruddy/bitcoin,Xekyo/bitcoin,MeshCollider/bitcoin,JeremyRubin/bitcoin,particl/particl-core,kallewoof/bitcoin,namecoin/namecoin-core,rnicoll/dogecoin,sstone/bitcoin,sipsorcery/bitcoin,namecoin/namecoin-core,jonasschnelli/bitcoin,GroestlCoin/GroestlCoin,MeshCollider/bitcoin,prusnak/bitcoin,ElementsProject/elements,rnicoll/bitcoin,AkioNak/bitcoin,instagibbs/bitcoin,qtumproject/qtum,namecoin/namecoin-core,Xekyo/bitcoin,qtumproject/qtum,jlopp/statoshi,mm-s/bitcoin,instagibbs/bitcoin,bitcoin/bitcoin,EthanHeilman/bitcoin,tecnovert/particl-core,jambolo/bitcoin,ElementsProject/elements,namecoin/namecore,domob1812/namecore,Sjors/bitcoin,litecoin-project/litecoin,yenliangl/bitcoin,n1bor/bitcoin,fujicoin/fujicoin,GroestlCoin/bitcoin,Xekyo/bitcoin,MarcoFalke/bitcoin,EthanHeilman/bitcoin,domob1812/bitcoin,prusnak/bitcoin,n1bor/bitcoin,fujicoin/fujicoin,bitcoin/bitcoin,practicalswift/bitcoin,domob1812/bitcoin,kallewoof/bitcoin,GroestlCoin/GroestlCoin,ajtowns/bitcoin,rnicoll/bitcoin,dscotese/bitcoin,sipsorcery/bitcoin,ajtowns/bitcoin,mm-s/bitcoin,jnewbery/bitcoin,fanquake/bitcoin,apoelstra/bitcoin,yenliangl/bitcoin,rnicoll/bitcoin,kallewoof/bitcoin,lateminer/bitcoin,bitcoinsSG/bitcoin,domob1812/bitcoin,lateminer/bitcoin,rnicoll/dogecoin,bitcoinknots/bitcoin,dscotese/bitcoin,domob1812/bitcoin,cdecker/bitcoin,MeshCollider/bitcoin,cdecker/bitcoin,jnewbery/bitcoin,jamesob/bitcoin,jlopp/statoshi,MarcoFalke/bitcoin,jonasschnelli/bitcoin,achow101/bitcoin,mruddy/bitcoin,bitcoinknots/bitcoin,lateminer/bitcoin,Xekyo/bitcoin,GroestlCoin/bitcoin,pataquets/namecoin-core,fujicoin/fujicoin,prusnak/bitcoin,sstone/bitcoin,EthanHeilman/bitcoin,qtumproject/qtum,pataquets/namecoin-core,sipsorcery/bitcoin,jlopp/statoshi,anditto/bitcoin,domob1812/bitcoin,domob1812/namecore,jamesob/bitcoin,jnewbery/bitcoin,instagibbs/bitcoin,AkioNak/bitcoin,fanquake/bitcoin,namecoin/namecore,jnewbery/bitcoin,achow101/bitcoin,apoelstra/bitcoin,MarcoFalke/bitcoin,sstone/bitcoin,apoelstra/bitcoin,bitcoinsSG/bitcoin,cdecker/bitcoin,achow101/bitcoin,bitcoinknots/bitcoin,litecoin-project/litecoin,pataquets/namecoin-core,namecoin/namecore,anditto/bitcoin,JeremyRubin/bitcoin,cdecker/bitcoin,tecnovert/particl-core,tecnovert/particl-core,EthanHeilman/bitcoin,domob1812/namecore,jambolo/bitcoin,alecalve/bitcoin,Sjors/bitcoin,ElementsProject/elements,yenliangl/bitcoin,practicalswift/bitcoin,MeshCollider/bitcoin,kallewoof/bitcoin,EthanHeilman/bitcoin,bitcoin/bitcoin,pstratem/bitcoin,alecalve/bitcoin,achow101/bitcoin,andreaskern/bitcoin,pstratem/bitcoin,tecnovert/particl-core,MarcoFalke/bitcoin,particl/particl-core,bitcoin/bitcoin,namecoin/namecore,namecoin/namecoin-core,MarcoFalke/bitcoin,mm-s/bitcoin,yenliangl/bitcoin,ElementsProject/elements,jambolo/bitcoin,achow101/bitcoin,domob1812/namecore,jonasschnelli/bitcoin,qtumproject/qtum,mruddy/bitcoin,jambolo/bitcoin,cdecker/bitcoin,jnewbery/bitcoin,dscotese/bitcoin,JeremyRubin/bitcoin,anditto/bitcoin,domob1812/namecore,apoelstra/bitcoin,prusnak/bitcoin,dscotese/bitcoin,GroestlCoin/bitcoin,andreaskern/bitcoin,prusnak/bitcoin,mm-s/bitcoin,MeshCollider/bitcoin,alecalve/bitcoin,jamesob/bitcoin,sipsorcery/bitcoin,rnicoll/dogecoin,ElementsProject/elements,instagibbs/bitcoin,Sjors/bitcoin,AkioNak/bitcoin,jambolo/bitcoin,rnicoll/bitcoin,namecoin/namecore,fujicoin/fujicoin,GroestlCoin/bitcoin,anditto/bitcoin,sstone/bitcoin,litecoin-project/litecoin,ajtowns/bitcoin,qtumproject/qtum,apoelstra/bitcoin,jambolo/bitcoin,dscotese/bitcoin,n1bor/bitcoin,lateminer/bitcoin,dscotese/bitcoin,anditto/bitcoin,n1bor/bitcoin,Xekyo/bitcoin,litecoin-project/litecoin,namecoin/namecoin-core,pstratem/bitcoin,bitcoin/bitcoin,practicalswift/bitcoin,bitcoin/bitcoin,Sjors/bitcoin,MeshCollider/bitcoin,tecnovert/particl-core,namecoin/namecore,sstone/bitcoin,rnicoll/dogecoin,andreaskern/bitcoin,bitcoinsSG/bitcoin,n1bor/bitcoin,jlopp/statoshi,GroestlCoin/GroestlCoin,pstratem/bitcoin,particl/particl-core,practicalswift/bitcoin,andreaskern/bitcoin,Sjors/bitcoin,pataquets/namecoin-core,bitcoinknots/bitcoin,ajtowns/bitcoin,rnicoll/bitcoin,litecoin-project/litecoin,EthanHeilman/bitcoin,yenliangl/bitcoin,GroestlCoin/bitcoin,jonasschnelli/bitcoin,rnicoll/bitcoin,GroestlCoin/GroestlCoin,domob1812/bitcoin,GroestlCoin/GroestlCoin,bitcoinsSG/bitcoin,practicalswift/bitcoin,litecoin-project/litecoin,domob1812/namecore,alecalve/bitcoin,apoelstra/bitcoin,bitcoinsSG/bitcoin,jonasschnelli/bitcoin,AkioNak/bitcoin,pataquets/namecoin-core,lateminer/bitcoin,lateminer/bitcoin,pataquets/namecoin-core,jamesob/bitcoin,yenliangl/bitcoin,bitcoinknots/bitcoin,jamesob/bitcoin,kallewoof/bitcoin,AkioNak/bitcoin,ajtowns/bitcoin,jlopp/statoshi,alecalve/bitcoin,fanquake/bitcoin,mruddy/bitcoin,mruddy/bitcoin,particl/particl-core,particl/particl-core,GroestlCoin/GroestlCoin,mruddy/bitcoin,n1bor/bitcoin,fujicoin/fujicoin,pstratem/bitcoin,ajtowns/bitcoin,fujicoin/fujicoin,AkioNak/bitcoin,jamesob/bitcoin,prusnak/bitcoin,rnicoll/dogecoin,GroestlCoin/bitcoin,namecoin/namecoin-core,ElementsProject/elements,tecnovert/particl-core,anditto/bitcoin,jlopp/statoshi,fanquake/bitcoin,fanquake/bitcoin,sipsorcery/bitcoin,mm-s/bitcoin,alecalve/bitcoin,pstratem/bitcoin,sstone/bitcoin,achow101/bitcoin,andreaskern/bitcoin