Remove obsolete build/config/ios/imageset.gni.

The file build/config/ios/imageset.gni was introduced to avoid
breaking the downstream build after the file rename. Remove it
as the downstream code has been fixed to use the new path.

Bug: 764286
Change-Id: I0dbaccca19f87017ed1a34ee7eeb37d94e1aaad9
Reviewed-on: https://chromium-review.googlesource.com/671384
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Eric Noyau <noyau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502837}

build/config/ios/imageset.gni

-# Copyright 2017 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.
-
-# TODO(crbug.com/764286): this is a forwarding .gni file to avoid breaking
-# the downstream repository that still reference this old path; remove it
-# once downstream has been fixed to use the new path.
-
-import("//build/config/ios/asset_catalog.gni")