blockchain: Remove unused median time calc func.
blockchain: Remove unused median time calc func.

This removes the CalcPastMedianTime since it is now exposed much more
efficiently via the MedianTime field of the BestState snapshot returned
from the BestSnapshot function.

tb00/btcd,dajohi/btcd,adiabat/btcd,adiabat/btcd,dajohi/btcd,tb00/btcd,Roasbeef/btcd,davecgh/btcd,aakselrod/btcd,jongillham/btcd,btcsuite/btcd,dajohi/btcd,cqswxj2013/btcd,davecgh/btcd,cqswxj2013/btcd,ltcsuite/ltcd,Stash-Crypto/btcd,ppcsuite/ppcd,jongillham/btcd,Stash-Crypto/btcd,ltcsuite/ltcd,wallclockbuilder/btcd,ppcsuite/ppcd,wallclockbuilder/btcd,Roasbeef/btcd,btcsuite/btcd,aakselrod/btcd