tty: Removing the deprecated function tty_vhangup_locked()

The function tty_vhangup_locked() was deprecated, removed it
from the tty.h also.

Signed-off-by: Liu, Chuansheng <chuansheng.liu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>