Revert "Discard invalid transaction" warning logs (#6137)

* Revert "Log missing chain head as warning, not trace (#6127)"
* leave chain head at warning, just revert the discard warning

Signed-off-by: garyschulte <garyschulte@gmail.com>

ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/transactions/TransactionPool.java

@@ -273,7 +273,7 @@ public class TransactionPool implements BlockAddedObserver {
         return ValidationResult.invalid(rejectReason);
       }
     } else {
-      LOG.atWarn()
+      LOG.atTrace()
           .setMessage("Discard invalid transaction {}, reason {}")
           .addArgument(transaction::toTraceLog)
           .addArgument(validationResult.result::getInvalidReason)