Remove unused ChangeModifiedException class

Change I10c989392 updated PatchSetInserter to not throw this exception,
and there are no other places that throw or catch it.

Change-Id: I6313639de18b6beed561ce34e3b2bcb852393950

gerrit-server/src/main/java/com/google/gerrit/server/project/ChangeModifiedException.java

@@ -1,23 +0,0 @@
-// Copyright (C) 2013 The Android Open Source Project
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package com.google.gerrit.server.project;
-
-public class ChangeModifiedException extends InvalidChangeOperationException {
-  private static final long serialVersionUID = 1L;
-
-  public ChangeModifiedException(String msg) {
-    super(msg);
-  }
-}