tty: drop obsolete termios_locked comments

Drop comments about tty-driver termios_locked structures, which have
been outdated since commit fe6e29fd

 ("tty: simplify ktermios
allocation").

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>