kernel: Prune ksched.h of dead code

There was a ton of junk in this header.  Pare it down to just the
stuff actually used by code outside of sched.c, move the needed
internal stuff into sched.c itself, and drop everything else.

Note that (other than the tiny inlines that remain here in the header)
the scheduler interface exposed to the rest of the system is now
composed of just 12 functions.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>