tty: remove empty tty_operations::flush_buffer

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.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>