Drop facebook_user_infos and twitter_user_infos (#6970)

Data was migrated to user_associated_accounts in 208005f and 160d29b

db/post_migrate/20190205104116_drop_unused_auth_tables.rb

@@ -0,0 +1,18 @@
+require 'migration/table_dropper'
+
+class DropUnusedAuthTables < ActiveRecord::Migration[5.2]
+  def change
+    def up
+      %i{
+        facebook_user_infos
+        twitter_user_infos
+      }.each do |table|
+        Migration::TableDropper.execute_drop(table)
+      end
+    end
+
+    def down
+      raise ActiveRecord::IrreversibleMigration
+    end
+  end
+end