Remove mod-only user revert system (#4178).

The mass undo system from #4178 replaces this system.

Followup to f2dccf8cf.

app/controllers/user_reverts_controller.rb

@@ -1,15 +0,0 @@
-class UserRevertsController < ApplicationController
-  before_action :moderator_only
-
-  def new
-    @user = User.find(params[:user_id])
-  end
-
-  def create
-    user = User.find(params[:user_id])
-    revert = UserRevert.new(user.id)
-    revert.process
-    redirect_to(user_path(user.id))
-  end
-end
-