Removed the full width for posts with the member attribution flag

- Full width was  part of the member attribution flag
- It's messing with email clicks flag
- Removing for both to make things easier

refs https://github.com/TryGhost/Team/issues/1960

ghost/admin/app/routes/posts.js

@@ -123,8 +123,7 @@ export default class PostsRoute extends AuthenticatedRoute {
 
     buildRouteInfoMetadata() {
         return {
-            titleToken: 'Posts',
-            mainClasses: this.feature.get('memberAttribution') ? ['gh-main-fullwidth'] : null
+            titleToken: 'Posts'
         };
     }