Bug 1733631 - [ci] Stop keying 'mochitest-plain' variants by-platform, r=jmaher

This does not add or remove any tasks.

Differential Revision: https://phabricator.services.mozilla.com/D128798

taskcluster/ci/test/mochitest.yml

@@ -55,12 +55,12 @@ mochitest-plain:
     schedules-component: mochitest-plain
     loopback-video: true
     virtualization: virtual
-
     variants:
-        by-test-platform:
-            linux1804-64-qr/debug: ['webrender-sw', 'fission', 'fission-xorigin', 'wayland', 'socketprocess_networking']
-            android-em-7.0-x86_64-qr/debug: ['webrender-sw', 'fission', 'fission-xorigin']
-            default: ['fission', 'fission-xorigin']
+        - fission
+        - fission-xorigin
+        - socketprocess_networking
+        - wayland
+        - webrender-sw
     run-on-projects:
         by-variant:
             fission:
@@ -76,6 +76,19 @@ mochitest-plain:
                     (linux.*64|macosx.*64|windows10-64-2004)(-shippable)?-qr/(opt|debug): ['trunk']
                     linux.*64-ccov-qr/opt: ['mozilla-central']
                     default: []
+            socketprocess_networking:
+                by-test-platform:
+                    linux1804-64-qr/debug: built-projects
+                    default: []
+            wayland:
+                by-test-platform:
+                    linux1804-64-qr/debug: built-projects
+                    default: []
+            webrender-sw:
+                by-test-platform:
+                    android-em-7.0-x86_64-qr/debug: built-projects
+                    linux1804-64-qr/debug: built-projects
+                    default: []
             default:
                 by-test-platform:
                     .*-tsan-qr/opt: ['trunk']