tty: Move tty_ldisc_kill()

In preparation for destroying the line discipline instance on hangup,
move tty_ldisc_kill() to eliminate needless forward declarations.
No functional change.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>