PERF: Remove foreign keys from bookmarks (#9837)

As a general rule we do not add FKs in Discourse and instead rely on consistency checks at weekly/daily intervals

db/post_migrate/20200520001619_remove_fks_from_bookmarks.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class RemoveFksFromBookmarks < ActiveRecord::Migration[6.0]
+  def change
+    remove_foreign_key :bookmarks, :topics
+    remove_foreign_key :bookmarks, :posts
+    remove_foreign_key :bookmarks, :users
+  end
+end