sched: Remove obsolete comment from scheduler_tick()

scheduler_tick() is no longer called by fork code - this got discarded
a long time ago by commit bc947631d1d532 ("sched: improve efficiency
of sched_fork()").

So, remove the comment which still claims otherwise.

Signed-off-by: Rakib Mullick <rakib.mullick@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/BANLkTimO4iGP0QpaHO1HHF1QOnVcQpc0cw@mail.gmail.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>