trivial: remove unused variable

Remove the unused variable "blockTmp" in CMerkleTx::SetMerkleBranch.  It
was previously used to read the block from disk if not provided as
argument, but is no longer needed.

src/wallet/wallet.cpp

@@ -3534,7 +3534,6 @@ CWalletKey::CWalletKey(int64_t nExpires)
 int CMerkleTx::SetMerkleBranch(const CBlockIndex* pindex, int posInBlock)
 {
     AssertLockHeld(cs_main);
-    CBlock blockTmp;
 
     // Update the tx's hashBlock
     hashBlock = pindex->GetBlockHash();