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/policies/favorite_policy.rb

@@ -4,6 +4,6 @@ class FavoritePolicy < ApplicationPolicy
   end
 
   def destroy?
-    !user.is_anonymous?
+    record.user_id == user.id
   end
 end

app/policies/post_policy.rb

@@ -81,7 +81,6 @@ class PostPolicy < ApplicationPolicy
     attributes += TagCategory.categories.map {|x| "tag_string_#{x}".to_sym}
     attributes += [:file_url, :large_file_url, :preview_file_url] if visible?
     attributes -= [:id, :md5] if !visible?
-    attributes -= [:fav_string]
     attributes
   end