Remove unneeded functions from memdb.
Remove unneeded functions from memdb.

This commit removes some functions from memdb which only existed to satify
the btcdb.Db interface, but have since been removed from the interface.

ppcsuite/ppcd,jrick/dcrd,Cleawing/btcd,FactomProject/btcd,Stash-Crypto/btcd,cjepson/btcd,jimmysong/btcd,dajohi/btcd,Cleawing/btcd,jcmincke/dcrd,tb00/btcd,jimmysong/btcd,pkothbauer/btcd,dan-da/btcd,ltcsuite/ltcd,CryptocurrencyCabal/htcd,DanielKrawisz/btcd,jongillham/btcd,adiabat/btcd,jrick/btcd,1d4Nf6/btcd,stevenroose/btcd,davecgh/btcd,soapboxsys/ombfullnode,ppcsuite/ppcd,dan-da/btcd,jongillham/btcd,btcsuite/btcd,stevenroose/btcd,ppcsuite/ppcd-old,cjepson/btcd,qchain/btcd,jrick/btcd,jrick/btcd,glv2/ppcd,aakselrod/btcd,ppcsuite/ppcd-old,wallclockbuilder/btcd,jcmincke/dcrd,jrick/dcrd,hectorj/btcd,danoctavian/btcd,qchain/btcd,runeaune/btcd,davecgh/btcd,cqswxj2013/btcd,danoctavian/btcd,pkothbauer/btcd,Roasbeef/btcd,FactomProject/btcd,cqswxj2013/btcd,jcmincke/dcrd,mathiasrw/btcd,DanielKrawisz/btcd,Roasbeef/btcd,tb00/btcd,wallclockbuilder/btcd,btcsuitereleases/btcd,Stash-Crypto/btcd,toddfries/btcd,glv2/ppcd,mathiasrw/btcd,adiabat/btcd,decred/dcrd,BitGo/prova,sysbot/btcd,ltcsuite/ltcd,Dirbaio/btcd,dajohi/btcd,toddfries/btcd,runeaune/btcd,btcsuite/btcd,Dirbaio/btcd,1d4Nf6/btcd,decred/dcrd,btcsuitereleases/btcd,soapboxsys/ombfullnode,dajohi/btcd,aakselrod/btcd,Dirbaio/btcd,sysbot/btcd,hectorj/btcd,CryptocurrencyCabal/htcd,BitGo/prova