DockedMegaMenu: Clean up toggle and old code (#81878)

* remove toggle

* remove code not behind toggle

* remove old MegaMenu

* rename DockedMegaMenu -> MegaMenu and clean up go code

* fix backend test

* run yarn i18n:extract

* fix some unit tests

* fix remaining unit tests

* fix remaining e2e/unit tests

public/locales/en-US/grafana.json

@@ -883,9 +883,6 @@
       "subtitle": "Manage preferences across an organization",
       "title": "Default preferences"
     },
-    "performance-testing": {
-      "title": "Performance testing"
-    },
     "playlists": {
       "subtitle": "Groups of dashboards that are displayed in a sequence",
       "title": "Playlists"