kernel: Further unify _reschedule APIs

Now that other work has eliminated the two cases where we had to do a
reschedule "but yield even if we are cooperative", we can squash both
down to a single _reschedule() function which does almost exactly what
legacy _Swap() did, but wrapped as a proper scheduler API.

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

kernel/mem_slab.c

@@ -127,5 +127,5 @@ void k_mem_slab_free(struct k_mem_slab *slab, void **mem)
 		slab->num_used--;
 	}
 
-	_reschedule_noyield(key);
+	_reschedule(key);
 }