tty: drop unused alt_speed from tty_struct

Drop the now unused alt_speed field from struct tty_struct.

Setting an alt_speed using the ASYNC_SPD flags has been deprecated since
v2.1.69, and has been broken for all tty drivers but serial-core since
v3.10 and commit 6865ff22 ("TTY: do not warn about setting speed via
SPD_*") without anyone noticing.

Note that serial-core still supports changing speed using TIOCSSERIAL
and SPD flags (including "alt-speeds"), but also warns about it being
deprecated since pre-git.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/tty.h

@@ -316,7 +316,6 @@ struct tty_struct {
 
 	struct tty_struct *link;
 	struct fasync_struct *fasync;
-	int alt_speed;		/* For magic substitution of 38400 bps */
 	wait_queue_head_t write_wait;
 	wait_queue_head_t read_wait;
 	struct work_struct hangup_work;