Bug 1563625 - Run Android 7.0/debug mochitests as tier 1; r=snorp

Debug mochitests are running as well as opt now, and sufficient for tier 1.

Differential Revision: https://phabricator.services.mozilla.com/D37016

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

taskcluster/ci/test/mochitest.yml

@@ -51,7 +51,6 @@ mochitest:
     loopback-video: true
     tier:
         by-test-platform:
-            android-em-7.*/debug: 2
             windows10-aarch64.*: 2
             default: default
     instance-size: