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).

db/populate.rb

@@ -186,8 +186,8 @@ if Favorite.count == 0
 
   Post.order("random()").limit(50).each do |post|
     user = User.order("random()").first
-    post.add_favorite!(user)
-    post.add_favorite!(CurrentUser.user)
+    Favorite.create!(post: post, user: user)
+    Favorite.create!(post: post, user: Currentuser.user)
   end
 else
   puts "Skipping favorites"