kernel: Remove abort and join implementation (UNBISECTABLE)

THIS COMMIT DELIBERATELY BREAKS BISECTABILITY FOR EASE OF REVIEW.
SKIP IF YOU LAND HERE.

Remove the existing implementatoin of k_thread_abort(),
k_thread_join(), and the attendant facilities in the thread subsystem
and idle thread that support them.

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