message: [wallet] restore coinbase check in SubmitMemoryPoolAndRelay()
  This check doesn't change mempool acceptance/relay behaviour, but reduces log spam.
diff: @@ -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());
  