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/miner.cpp

@@ -148,7 +148,6 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
         CBlockIndex* pindexPrev = chainActive.Tip();
         const int nHeight = pindexPrev->nHeight + 1;
         pblock->nTime = GetAdjustedTime();
-        const int64_t nMedianTimePast = pindexPrev->GetMedianTimePast();
         CCoinsViewCache view(pcoinsTip);
 
         // Priority order to process transactions
@@ -163,12 +162,7 @@ CBlockTemplate* CreateNewBlock(const CScript& scriptPubKeyIn)
              mi != mempool.mapTx.end(); ++mi)
         {
             const CTransaction& tx = mi->GetTx();
-
-            int64_t nLockTimeCutoff = (STANDARD_LOCKTIME_VERIFY_FLAGS & LOCKTIME_MEDIAN_TIME_PAST)
-                                    ? nMedianTimePast
-                                    : pblock->GetBlockTime();
-
-            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, nLockTimeCutoff))
+            if (tx.IsCoinBase() || !IsFinalTx(tx, nHeight, pblock->nTime))
                 continue;
 
             COrphan* porphan = NULL;