Remove some unused history resources on iOS

ios/chrome/app/resources/history/alert_overlay.css

Index: ios/chrome/app/resources/history/alert_overlay.css
diff --git a/ios/chrome/app/resources/history/alert_overlay.css b/ios/chrome/app/resources/history/alert_overlay.css
deleted file mode 100644
index 5c6409a6945a5b026a2b2cf76002994a86b55ed4..0000000000000000000000000000000000000000
--- a/ios/chrome/app/resources/history/alert_overlay.css
+++ /dev/null
@@ -1,20 +0,0 @@
-/* Copyright 2013 The Chromium Authors. All rights reserved.
- * Use of this source code is governed by a BSD-style license that can be
- * found in the LICENSE file. */
-
-.overlay .page .button-strip > button {
-  min-width: 7em;
-}
-
-.overlay {
-  z-index: 4;
-}
-
-#alertOverlayMessage {
-  width: 400px;
-}
-
-#alertOverlayMessage {
-  white-space: pre-wrap;
-  word-wrap: break-word;
-}