Documentation: tty: n_gsm, delete "Additional Documentation"

All of the URLs give 404. There is a little point to preserve them. If
really need be, we could try archive.org if the documents are not there
by a chance.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220411110143.10019-6-jslaby@suse.cz


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