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

The mass undo system from #4178 replaces this system.

Followup to f2dccf8cf.

test/unit/user_revert_test.rb

@@ -1,44 +0,0 @@
-require 'test_helper'
-
-class UserRevertTest < ActiveSupport::TestCase
-  context "Reverting a user's changes" do
-    setup do
-      @creator = create(:user)
-      @user = create(:user)
-
-      CurrentUser.scoped(@creator) do
-        @parent = create(:post)
-        @post = create(:post, :tag_string => "aaa bbb ccc", :rating => "q", :source => "xyz")
-      end
-
-      @post.stubs(:merge_version?).returns(false)
-      CurrentUser.scoped(@user) do
-        @post.update(:tag_string => "bbb ccc xxx", :source => "", :rating => "e")
-      end
-    end
-
-    subject { UserRevert.new(@user.id) }
-
-    should "have the correct data" do
-      assert_equal("bbb ccc xxx", @post.tag_string)
-      assert_equal("", @post.source)
-      assert_equal("e", @post.rating)
-    end
-
-    context "when processed" do
-      setup do
-        CurrentUser.as(@user) do
-          subject.process
-        end
-        @post.reload
-      end
-
-      should "revert the user's changes" do
-        assert_equal("aaa bbb ccc", @post.tag_string)
-        assert_equal("xyz", @post.source)
-        assert_equal("q", @post.rating)
-      end
-    end
-
-  end
-end