Clean up CMerkleTx::SetMerkleBranch.

The case SetMerkleBranch(NULL) was never actually used, and thus the
involved code (loading the block from disk) can be removed and the
implementation simplified.

src/wallet.h

@@ -500,7 +500,7 @@ public:
     )
 
 
-    int SetMerkleBranch(const CBlock* pblock=NULL);
+    int SetMerkleBranch(const CBlock& block);
 
     // Return depth of transaction in blockchain:
     // -1  : not in blockchain, and not in memory pool (conflicted transaction)