SAM3/4 serial: Same supersitituous change as for SAMA5

arch/arm/src/sam34/sam_serial.c

@@ -1081,13 +1081,12 @@ static void up_txint(struct uart_dev_s *dev, bool enable)
 #ifndef CONFIG_SUPPRESS_SERIAL_INTS
       up_serialout(priv, SAM_UART_IER_OFFSET, UART_INT_TXRDY);
 
-#  if 0 /* Seems to be unnecessary */
       /* Fake a TX interrupt here by just calling uart_xmitchars() with
        * interrupts disabled (note this may recurse).
        */
 
       uart_xmitchars(dev);
-#  endif
+
 #endif
     }
   else