Most of the structure predeclarations were at the head of sched.h, but not
all of them - there were a number of lines spread around sched.h, in
random places.

Move them to the head, and also sort them alphabetically.

Remove unused entries.
Acked-by: NLinus Torvalds <torvalds@linux-foundation.org>
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: NIngo Molnar <mingo@kernel.org>