Bug 1733631 - [ci] Stop setting 'variants' in defaults section of reftest.yml, r=jmaher

Differential Revision: https://phabricator.services.mozilla.com/D128965

taskcluster/ci/test/reftest.yml

@@ -10,7 +10,6 @@ job-defaults:
             android-em-7.*: geckoview-androidTest.apk
             android-hw-.*: geckoview-androidTest.apk
             default: null
-    variants: ['fission']
     python-3: true
     tier:
         by-variant:
@@ -96,6 +95,7 @@ crashtest-qr:
     treeherder-symbol: R(C)
     chunks: 1
     e10s: true
+    variants: ['fission']
     run-on-projects:
         by-variant:
             fission: []
@@ -129,6 +129,7 @@ jsreftest:
             macosx.*64-ccov.*/.*: 7200
             linux.*64-ccov.*/.*: 7200
             default: 3600
+    variants: ['fission']
     run-on-projects:
         by-variant:
             fission: []