doc: remove passing `--disable-external-signer` in OpenBSD build guide
doc: remove passing `--disable-external-signer` in OpenBSD build guide

Since the Boost.Process usage check was added to the build system
(commit abc057c6030b2a0ddab46835a7801054da677781), passing the option
`--disable-external-signer` explicitly is not needed anymore on OpenBSD;
The configure script will automatically detect that including
<boost/process.hpp> leads to a compile error and disable external signer
support accordingly.

sipsorcery/bitcoin,Xekyo/bitcoin,instagibbs/bitcoin,kallewoof/bitcoin,lateminer/bitcoin,fujicoin/fujicoin,AkioNak/bitcoin,sipsorcery/bitcoin,andreaskern/bitcoin,sstone/bitcoin,lateminer/bitcoin,domob1812/bitcoin,AkioNak/bitcoin,domob1812/bitcoin,tecnovert/particl-core,jamesob/bitcoin,Xekyo/bitcoin,andreaskern/bitcoin,GroestlCoin/GroestlCoin,sstone/bitcoin,tecnovert/particl-core,domob1812/bitcoin,fanquake/bitcoin,instagibbs/bitcoin,lateminer/bitcoin,namecoin/namecore,bitcoinsSG/bitcoin,bitcoin/bitcoin,GroestlCoin/GroestlCoin,andreaskern/bitcoin,particl/particl-core,jambolo/bitcoin,jambolo/bitcoin,fanquake/bitcoin,bitcoin/bitcoin,andreaskern/bitcoin,ajtowns/bitcoin,domob1812/bitcoin,tecnovert/particl-core,bitcoinsSG/bitcoin,jamesob/bitcoin,GroestlCoin/bitcoin,jambolo/bitcoin,jamesob/bitcoin,namecoin/namecore,GroestlCoin/GroestlCoin,lateminer/bitcoin,namecoin/namecore,bitcoin/bitcoin,kallewoof/bitcoin,tecnovert/particl-core,Xekyo/bitcoin,AkioNak/bitcoin,tecnovert/particl-core,bitcoinsSG/bitcoin,namecoin/namecoin-core,namecoin/namecoin-core,GroestlCoin/bitcoin,sstone/bitcoin,ajtowns/bitcoin,sipsorcery/bitcoin,instagibbs/bitcoin,namecoin/namecore,particl/particl-core,GroestlCoin/bitcoin,sipsorcery/bitcoin,jambolo/bitcoin,mruddy/bitcoin,sstone/bitcoin,particl/particl-core,jambolo/bitcoin,kallewoof/bitcoin,namecoin/namecoin-core,sstone/bitcoin,bitcoin/bitcoin,jamesob/bitcoin,particl/particl-core,ajtowns/bitcoin,AkioNak/bitcoin,namecoin/namecoin-core,fanquake/bitcoin,domob1812/bitcoin,ajtowns/bitcoin,mruddy/bitcoin,namecoin/namecoin-core,jamesob/bitcoin,kallewoof/bitcoin,namecoin/namecore,sipsorcery/bitcoin,fujicoin/fujicoin,ajtowns/bitcoin,AkioNak/bitcoin,kallewoof/bitcoin,AkioNak/bitcoin,GroestlCoin/GroestlCoin,bitcoinsSG/bitcoin,fujicoin/fujicoin,Xekyo/bitcoin,mruddy/bitcoin,andreaskern/bitcoin,jamesob/bitcoin,bitcoin/bitcoin,andreaskern/bitcoin,fujicoin/fujicoin,domob1812/bitcoin,lateminer/bitcoin,sstone/bitcoin,bitcoin/bitcoin,mruddy/bitcoin,ajtowns/bitcoin,mruddy/bitcoin,instagibbs/bitcoin,GroestlCoin/bitcoin,kallewoof/bitcoin,GroestlCoin/bitcoin,tecnovert/particl-core,fanquake/bitcoin,fanquake/bitcoin,sipsorcery/bitcoin,GroestlCoin/GroestlCoin,instagibbs/bitcoin,mruddy/bitcoin,Xekyo/bitcoin,fujicoin/fujicoin,GroestlCoin/bitcoin,bitcoinsSG/bitcoin,fujicoin/fujicoin,GroestlCoin/GroestlCoin,particl/particl-core,jambolo/bitcoin,namecoin/namecoin-core,Xekyo/bitcoin,namecoin/namecore,fanquake/bitcoin,bitcoinsSG/bitcoin,lateminer/bitcoin,particl/particl-core,instagibbs/bitcoin