[wallet] restore coinbase check in SubmitMemoryPoolAndRelay()

This check doesn't change mempool acceptance/relay behaviour, but reduces log spam.

Author: jnewbery

src/wallet/wallet.cpp

@@ -2160,6 +2160,9 @@ bool CWalletTx::SubmitMemoryPoolAndRelay(std::string& err_string, bool relay)
     if (!pwallet->GetBroadcastTransactions()) return false;
     // Don't relay abandoned transactions
     if (isAbandoned()) return false;
+    // Don't try to submit coinbase transactions. These would fail anyway but would
+    // cause log spam.
+    if (IsCoinBase()) return false;
 
     // Submit transaction to mempool for relay
     pwallet->WalletLogPrintf("Submitting wtx %s to mempool for relay\n", GetHash().ToString());