sched/core: Remove unnecessary #include headers

Over the years sched/core.c accumulated over 50 #include lines,
40 of which are superfluous. (!)

Removing them decreases the preprocessed .c file (.i) size noticeably:

            triton:~/tip> wc -l kernel/sched/core.i

  Before:   76387 kernel/sched/core.i
  After:    75896 kernel/sched/core.i

Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>