Revert "[autotest] Threaded asynchronous task execution on drones." 

Problems with the retry decorator and localhost threads.

This reverts commit 0933899b0dd1320e90e06025cced8096aed44908.

Change-Id: I99318b4bdf4c11e9c4e5181c4ff5b1bdcdcbb89c
Reviewed-on: https://chromium-review.googlesource.com/207038
Reviewed-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Prashanth B <beeps@chromium.org>
Tested-by: Prashanth B <beeps@chromium.org>
diff --git a/scheduler/monitor_db.py b/scheduler/monitor_db.py
index ddcb028..4860859 100755
--- a/scheduler/monitor_db.py
+++ b/scheduler/monitor_db.py
@@ -299,8 +299,8 @@
         timer = stats.Timer('scheduler.tick')
         self._log_tick_msg('Calling new tick, starting garbage collection().')
         self._garbage_collection()
-        self._log_tick_msg('Calling _drone_manager.trigger_refresh().')
-        _drone_manager.trigger_refresh()
+        self._log_tick_msg('Calling _drone_manager.refresh().')
+        _drone_manager.refresh()
         self._log_tick_msg('Calling _run_cleanup().')
         self._run_cleanup()
         self._log_tick_msg('Calling _find_aborting().')
@@ -317,8 +317,6 @@
         self._schedule_special_tasks()
         self._log_tick_msg('Calling _schedule_new_jobs().')
         self._schedule_new_jobs()
-        self._log_tick_msg('Calling _drone_manager.sync_refresh().')
-        _drone_manager.sync_refresh()
         self._log_tick_msg('Calling _handle_agents().')
         self._handle_agents()
         self._log_tick_msg('Calling _host_scheduler.tick().')