Bug 1601981 - Make wpt fission tests tier 2, r=ahal,neha

This also reduces the configuration to just the -qr subset of the
configuration that runs for mochitest -fis.

Differential Revision: https://phabricator.services.mozilla.com/D56197

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

taskcluster/ci/test/web-platform.yml

@@ -18,16 +18,9 @@ job-defaults:
         by-test-platform:
             linux64-qr/debug: ['trunk']
             linux64-qr/opt: ['mozilla-central']
-            linux64/.*: ['mozilla-central']
-            windows10-64(-qr)?/opt: ['mozilla-central']
+            windows10-64-qr/opt: ['mozilla-central']
             default: []
-    fission-tier:
-        by-test-platform:
-            linux64-qr/debug: 3
-            linux64-qr/opt: 3
-            linux64/.*: 3
-            windows10-64(-qr)?/opt: 3
-            default: 2
+    fission-tier: 2
     mozharness:
         script: web_platform_tests.py
         config: