Revert "Add rules--presently disabled--for using GetMedianTimePast as endpoint for lock-time calculations"

This reverts commit 9d55050773d57c0e12005e524f2e54d9e622c6e2.

As noted by Luke-Jr, under some conditions this will accept transactions which are invalid by the network
rules. This happens when the current block time is head of the median time past and a transaction's
locktime is in the middle.

This could be addressed by changing the rule to MAX(this_block_time, MTP+offset) but this solution and
the particular offset used deserve some consideration.