posts: stop updating fav_string attribute.

Stop updating the fav_string attribute on posts. The column still exists
on the table, but is no longer used or updated.

Like the pool_string in 7d503f08, the fav_string was used in the past to
facilitate `fav:X` searches. Posts had a hidden fav_string column that
contained a list of every user who favorited the post. These were
treated like fake hidden tags on the post so that a search for `fav:X`
was treated like a tag search.

The fav_string attribute has been unused for search purposes for a while
now. It was only kept because of technicalities that required
departitioning the favorites table first (340e1008e) before it could be
removed. Basically, removing favorites with `@favorite.destroy` was
slow because Rails always deletes object by ID, but we didn't have an
index on favorites.id, and we couldn't easily add one until the
favorites table was departitioned.

Fixes #4652. See https://github.com/danbooru/danbooru/issues/4652#issuecomment-754993802
for more discussion of issues caused by the fav_string (in short: write
amplification, post table bloat, and favorite inconsistency problems).

app/logical/post_query_builder.rb

@@ -441,7 +441,7 @@ class PostQueryBuilder
     user = User.find_by_name(username)
 
     if user.present? && Pundit.policy!(current_user, user).can_see_favorites?
-      Post.joins(:favorites).merge(Favorite.for_user(user.id)).order("favorites.id DESC")
+      Post.joins(:favorites).merge(Favorite.where(user: user)).order("favorites.id DESC")
     else
       Post.none
     end