tty: remove unused tty_driver->termios_locked

This field is unused since 2.6.28 (commit fe6e29fd: "tty: simplify
ktermios allocation", to be exact)

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>