Bug 1519850 - [marionette] Bump Mn test jobs on Android to Tier1. r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D16555

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/marionette.yml

@@ -30,10 +30,6 @@ marionette:
         by-test-platform:
             android-em.*: xlarge
             default: default
-    tier:
-        by-test-platform:
-            android-em.*: 2
-            default: default
     chunks:
         by-test-platform:
             android-em.*: 10