Doc: tty.txt: remove mention of the BKL

The BKL is long gone, no need to kill the dead.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/serial/tty.txt

@@ -4,9 +4,6 @@
 Your guide to the ancient and twisted locking policies of the tty layer and
 the warped logic behind them. Beware all ye who read on.
 
-FIXME: still need to work out the full set of BKL assumptions and document
-them so they can eventually be killed off.
-
 
 Line Discipline
 ---------------