sched/core: Move, sort and clean up <linux/sched.h> structure predeclarations

sched/core: Move, sort and clean up <linux/sched.h> structure predeclarations

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: Linus Torvalds <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>