Get rid of system_export.h, etc.

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")
 
 # Depend on this target to use the types etc defined in the system without
 # linking against a specific implementation of the system. To link against a
 # particular implementation, use the :for_component or
 # :for_shared_library targets, depending on the type of target you are.
 mojo_sdk_source_set("system") {
   sources = [
     "buffer.h",
     "core.h",
     "data_pipe.h",
     "functions.h",
     "macros.h",
     "message_pipe.h",
-    "system_export.h",
     "types.h",
   ]
 }
 
 # In an is_component_build build, everything can link against //mojo/edk/system
 # because it is built as a shared library. However, in a static build,
 # //mojo/edk/system is linked into an executable (e.g., mojo_shell), and must be
 # injected into other shared libraries (i.e., Mojo Apps) that need the mojo
 # system API.
 #
@@ skipping 16 matching lines @@
 group("for_component") {
   public_deps = [
     ":system",
   ]
   if (is_component_build) {
     deps = [
       "../../../edk/system",
     ]
   }
 }