Bug 1403342 - default to -j none and do not optimize_target_tasks for try; r=ahal

With this in place, all `-j`obs will not run by default on try.  This will omit
such jobs in most try pushes even if files-changed matches. This is
unfortunate, but better than running them unconditionally.  Fuzzy selections,
and later `just try it` pushes, are the ultimate solution here.

With this change, a push with no try syntax or try_task_config.json will schedule
no tasks at all.

MozReview-Commit-ID: FGjqlDW1FT6

taskcluster/taskgraph/target_tasks.py

@@ -110,11 +110,9 @@ def target_tasks_try(full_task_graph, parameters):
     elif try_mode == 'try_option_syntax':
         return _try_option_syntax(full_task_graph, parameters)
     else:
-        # With no try mode, we would like to schedule everything (following
-        # run_on_projects) and let optimization trim it down.  But optimization
-        # isn't yet up to the task, so instead we use try_option_syntax with
-        # an empty message (which basically just schedules `-j`objs)
-        return _try_option_syntax(full_task_graph, parameters)
+        # With no try mode, we schedule nothing, allowing the user to add tasks
+        # later via treeherder.
+        return []
 
 
 @_target_task('default')