tty: remove unnecessary export of proc_clear_tty

All of the users of proc_clear_tty are compiled into the kernel so exporting
this symbol appears gratuitous.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>