tty: remove empty tty_operations::flush_buffer

tty_operations::flush_buffer is optional. If it doesn't exist, nothing
is called. So remove almost¹ empty flush_buffer implementations.

¹ capi had an useless pr_debug in it.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Link: https://lore.kernel.org/r/20210505091928.22010-31-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/isdn/capi/capi.c

@@ -1249,11 +1249,6 @@ static int capinc_tty_break_ctl(struct tty_struct *tty, int state)
 	return 0;
 }
 
-static void capinc_tty_flush_buffer(struct tty_struct *tty)
-{
-	pr_debug("capinc_tty_flush_buffer\n");
-}
-
 static void capinc_tty_set_ldisc(struct tty_struct *tty)
 {
 	pr_debug("capinc_tty_set_ldisc\n");
@@ -1279,7 +1274,6 @@ static const struct tty_operations capinc_ops = {
 	.start = capinc_tty_start,
 	.hangup = capinc_tty_hangup,
 	.break_ctl = capinc_tty_break_ctl,
-	.flush_buffer = capinc_tty_flush_buffer,
 	.set_ldisc = capinc_tty_set_ldisc,
 	.send_xchar = capinc_tty_send_xchar,
 	.install = capinc_tty_install,

drivers/s390/char/tty3270.c

@@ -1756,11 +1756,6 @@ tty3270_flush_chars(struct tty_struct *tty)
 	}
 }
 
-static void
-tty3270_flush_buffer(struct tty_struct *tty)
-{
-}
-
 /*
  * Check for visible/invisible input switches
  */
@@ -1881,7 +1876,6 @@ static const struct tty_operations tty3270_ops = {
 	.put_char = tty3270_put_char,
 	.flush_chars = tty3270_flush_chars,
 	.write_room = tty3270_write_room,
-	.flush_buffer = tty3270_flush_buffer,
 	.throttle = tty3270_throttle,
 	.unthrottle = tty3270_unthrottle,
 	.hangup = tty3270_hangup,