Remove extra nonce reset logic in IncrementExtraNonce

I assume this was added to prevent deanonymization, but that
doesn't matter anymore since its only used on regtest.

Author: TheBlueMatt

src/miner.cpp

@@ -589,13 +589,7 @@ void BlockAssembler::addPriorityTxs()
 
 void IncrementExtraNonce(CBlock* pblock, const CBlockIndex* pindexPrev, unsigned int& nExtraNonce)
 {
-    // Update nExtraNonce
-    static uint256 hashPrevBlock;
-    if (hashPrevBlock != pblock->hashPrevBlock)
-    {
-        nExtraNonce = 0;
-        hashPrevBlock = pblock->hashPrevBlock;
-    }
+    assert(pblock->hashPrevBlock == pindexPrev->GetBlockHash());
     ++nExtraNonce;
     unsigned int nHeight = pindexPrev->nHeight+1; // Height first in coinbase required for block.version=2
     CMutableTransaction txCoinbase(pblock->vtx[0]);