fix: remove unused properties

These properties have not yet been through the property onboarding
process, and so should not yet be exposed. Plus they were never being
set.

BREAKING CHANGE: `Post.thumbnailHeight` and `Post.thumbnailWidth` have
been temporarily removed. They were not yet supposed to be exposed, but
they slipped through the cracks.

src/objects/post.ts

@@ -283,8 +283,8 @@ export class Post extends Voteable implements PostData {
   subredditSubscribers: number;
   // suggestedSort: Maybe<Sort>;
   // thumbnail: string;
-  thumbnailHeight?: number;
-  thumbnailWidth?: number;
+  // thumbnailHeight?: number;
+  // thumbnailWidth?: number;
   title: string;
   upvoteRatio: number;
   url: string;