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

This reverts commit 9d55050773.

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.

src/consensus/consensus.h

@@ -13,10 +13,4 @@ static const unsigned int MAX_BLOCK_SIGOPS = MAX_BLOCK_SIZE/50;
 /** Coinbase transaction outputs can only be spent after this number of new blocks (network rule) */
 static const int COINBASE_MATURITY = 100;
 
-/** Flags for LockTime() */
-enum {
-    /* Use GetMedianTimePast() instead of nTime for end point timestamp. */
-    LOCKTIME_MEDIAN_TIME_PAST = (1 << 1),
-};
-
 #endif // BITCOIN_CONSENSUS_CONSENSUS_H