blockchain: Remove unused min memory nodes field.
blockchain: Remove unused min memory nodes field.

This removes the minMemoryNodes field from the BlockChain struct since
it is no longer used anywhere.

aakselrod/btcd,Roasbeef/btcd,davecgh/btcd,tb00/btcd,dajohi/btcd,aakselrod/btcd,ltcsuite/ltcd,davecgh/btcd,btcsuite/btcd,Roasbeef/btcd,dajohi/btcd,ltcsuite/ltcd,btcsuite/btcd,ppcsuite/ppcd,Stash-Crypto/btcd,Stash-Crypto/btcd,tb00/btcd,ppcsuite/ppcd,wallclockbuilder/btcd,dajohi/btcd,wallclockbuilder/btcd