Bug 1640278 - Remove 'try' from checking run-on-projects for try-syntax. r=bhearsum

This could go further and test for m-c or the trunk/integration aliases, etc. but I opted for not letting this increase try-syntax jobs.

Differential Revision: https://phabricator.services.mozilla.com/D81756

taskcluster/taskgraph/try_option_syntax.py

@@ -569,7 +569,7 @@ class TryOptionSyntax(object):
         attr = task.attributes.get
 
         def check_run_on_projects():
-            return set(['try', 'all']) & set(attr('run_on_projects', []))
+            return {'all'} & set(attr('run_on_projects', []))
 
         # Don't schedule fission tests when try option syntax is used
         if attr('unittest_variant') == 'fission':