Revert "core: add missing SCHEDULING_{PUSH,POP} around pop_task()"

This reverts commit 468f9ac22c48c6e36bb02d248a31ee224bda1c69.

This has introduced a huge performance regression when using
STARPU_GENERATE_TRACE=1. For example with the tag_example test
case, the total execution time has been increased by a factor
of 3 and the number of generated lines in trace.rec was ~85M
and it is now at ~21M.

src/core/sched_policy.c

@@ -805,9 +805,7 @@ pick:
 			{
 				if (sched_ctx->sched_policy && sched_ctx->sched_policy->pop_task)
 				{
-					_STARPU_TRACE_WORKER_SCHEDULING_PUSH;
 					task = sched_ctx->sched_policy->pop_task(sched_ctx->id);
-					_STARPU_TRACE_WORKER_SCHEDULING_POP;
 					_starpu_pop_task_end(task);
 				}
 			}