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/main.h

@@ -308,10 +308,8 @@ bool IsFinalTx(const CTransaction &tx, int nBlockHeight, int64_t nBlockTime);
  * Check if transaction will be final in the next block to be created.
  *
  * Calls IsFinalTx() with current block height and appropriate block time.
- *
- * See consensus/consensus.h for flag definitions.
  */
-bool CheckFinalTx(const CTransaction &tx, int flags = -1);
+bool CheckFinalTx(const CTransaction &tx);
 
 /** 
  * Closure representing one script verification