Deleted collections labs flag

ref https://linear.app/ghost/issue/ENG-1805/remove-collections-code

- I'm working on deleting the collections code, and there is a LOT
- to break it down, I've already removed the Admin code so there's no
  frontend to play around with
- now, this commit removes the flag and deletes the associated tests so
  we can be sure the functionality is actually disabled
- after this, I can start deleting the backend code

ghost/core/core/shared/labs.js

@@ -15,7 +15,6 @@ const messages = {
 // flags in this list always return `true`, allows quick global enable prior to full flag removal
 const GA_FEATURES = [
     'audienceFeedback',
-    'collections',
     'i18n',
     'themeErrorsNotification',
     'outboundLinkTagging',

ghost/core/test/e2e-api/admin/__snapshots__/config.test.js.snap

@@ -15,7 +15,6 @@ Object {
       "adminXDemo": true,
       "announcementBar": true,
       "audienceFeedback": true,
-      "collections": true,
       "collectionsCard": true,
       "commentImprovements": true,
       "contentVisibility": true,