remove superfluous ABT_thread_yield() call

improves ops/s by 2-3% in sequntial benchmarks

src/margo-core.c

@@ -1973,10 +1973,6 @@ void __margo_hg_progress_fn(void* foo)
             ret = margo_internal_trigger(mid, 0, 1, &actual_count);
         } while ((ret == HG_SUCCESS) && actual_count
                  && !mid->hg_progress_shutdown_flag);
-        /* once we have processed callbacks, give the ES an opportunity to
-         * run other ULTs if it needs to.
-         */
-        ABT_thread_yield();
 
         /* Check to see if there are any runnable ULTs in the pool now.  If
          * so, then we yield here to allow them a chance to execute.