xen/serial: drop stale SERIAL_NEVER_DROP_CHARS

Left over from 2754857001 ("Bring back console_start_log_everything()
as a milder alternative to"), which dropped the sole use only a few weeks
after its introduction.

Signed-off-by: Denis Mukhin <dmukhin@ford.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/char/serial.c

@@ -15,9 +15,6 @@
 
 #include <asm/processor.h>
 
-/* Never drop characters, even if the async transmit buffer fills. */
-/* #define SERIAL_NEVER_DROP_CHARS 1 */
-
 unsigned int __ro_after_init serial_txbufsz = CONFIG_SERIAL_TX_BUFSIZE;
 size_param("serial_tx_buffer", serial_txbufsz);