tty: unexport tty_ldisc_release

Initially, tty_ldisc_release() was exported for speakup (spk_tty) while
in staging. Later, the call to this function was removed as it was bogus
anyway.

Remove the export now.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210914091134.17426-1-jslaby@suse.cz


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