MAINTAINERS, sched: Drop PREEMPTIBLE KERNEL section entry

The PREEMPTIBLE KERNEL section entry seems quite outdated:

Robert Love is not actively maintaining the file anymore, nor a recorded
contributor to the files in the PREEMPTIBLE KERNEL section for the last
few years.

The mailing list kpreempt-tech@lists.sourceforge.net does not exist
anymore; the website just points to some very old patches for v2.4/v2.5.

So, let's delete the PREEMPTIBLE KERNEL section entry and clean this up:

- Documentation/preempt-locking.txt is not modified much anyway, and the
  changes in that file are generally maintained by Jonathan Corbet. So,
  we do not need to explicitly mention Documentation/preempt-locking.txt
  in MAINTAINERS.

- include/linux/preempt.h is maintained by Peter and Ingo, so we simply
  add that file to the SCHEDULER section entry.

I got directed to this issue, as I could not subscribe to the outdated
mailing list address and decided to investigate and then cleaned this up.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Robert Love <rlove@rlove.org>
Cc: Robert Love <rml@tech9.net>
Cc: Robert Love <robert.w.love@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20190112060613.7115-1-lukas.bulwahn@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>