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).

test/unit/user_deletion_test.rb

@@ -69,13 +69,12 @@ class UserDeletionTest < ActiveSupport::TestCase
 
     should "remove any favorites" do
       @post = create(:post)
-      Favorite.add(post: @post, user: @user)
+      Favorite.create!(post: @post, user: @user)
 
       perform_enqueued_jobs { @deletion.delete! }
 
       assert_equal(0, Favorite.count)
-      assert_equal("", @post.reload.fav_string)
-      assert_equal(0, @post.fav_count)
+      assert_equal(0, @post.reload.fav_count)
     end
   end
 end