Remove the //mojo/public/c/system:for_shared_library target.

mojo/public/c/system/BUILD.gn

 # Copyright 2014 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.
 
 import("../../mojo_sdk.gni")
 
 # This target merely declares the Mojo system API's types, functions, etc. To
 # use the functions, you must also depend on a specific implementation (e.g.,
 # ../../platform/native:system).
 mojo_sdk_source_set("system") {
   sources = [
     "buffer.h",
     "core.h",
     "data_pipe.h",
     "functions.h",
     "macros.h",
     "message_pipe.h",
     "types.h",
   ]
 }
-
-# TODO(vtl): Remove this.
-group("for_shared_library") {
-  public_deps = [
-    ":system",
-  ]
-  deps = [
-    "../../platform/native:system",
-  ]
-}