tty: synclink_gt: remove broken driver

tty: synclink_gt: remove broken driver

The synclink_gt driver was marked as broken in commit 426263d5fb40
("tty: synclink_gt: mark as BROKEN") in July 2023 because it had severe
structural problems and there had been no evidence of users since 2016.
Since then, no meaningful improvements have been made to the driver,
and it is unlikely that will ever happen due to the lack of interest.
Drop the driver and references to it in comments and documentation.
include/uapi/linux/synclink.h is also removed. The only use of this
header I have found is the linux-raw-sys Rust crate. It generates
bindings for all UAPI headers, but has a hardcoded list of headers and
ioctls, including this one, so that does not indicate that anyone is
using it. I have sent a pull request to remove the include and ioctl
definitions for this header (see the link below).

Link: https://github.com/sunfishcode/linux-raw-sys/pull/185
Signed-off-by: Ethan Nelson-Moore <enelsonmoore@gmail.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://patch.msgid.link/20260504031519.18877-1-enelsonmoore@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>