kernel: Remove legacy preemption checking

The metairq feature exposed the fact that all of our arch code (and a
few mistaken spots in the scheduler too) was trying to interpret
"preemptible" threads independently.

As of the scheduler rewrite, that logic is entirely within sched.c and
doing it externally is redundant.  And now that "cooperative" threads
can be preempted, it's wrong and produces test failures when used with
metairq threads.

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