Remove unused FML file export.h (#7926)

We should not be exporting anything from FML anyway.

ci/licenses_golden/licenses_flutter

@@ -128,7 +128,6 @@ FILE: ../../../flutter/fml/command_line.h
 FILE: ../../../flutter/fml/command_line_unittest.cc
 FILE: ../../../flutter/fml/compiler_specific.h
 FILE: ../../../flutter/fml/eintr_wrapper.h
-FILE: ../../../flutter/fml/export.h
 FILE: ../../../flutter/fml/file.cc
 FILE: ../../../flutter/fml/file.h
 FILE: ../../../flutter/fml/file_unittest.cc

fml/BUILD.gn

@@ -13,7 +13,6 @@ source_set("fml") {
     "command_line.h",
     "compiler_specific.h",
     "eintr_wrapper.h",
-    "export.h",
     "file.cc",
     "file.h",
     "icu_util.cc",

fml/export.h

-// Copyright 2013 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef FLUTTER_FML_EXPORT_H_
-#define FLUTTER_FML_EXPORT_H_
-
-#include "flutter/fml/build_config.h"
-
-#if OS_WIN
-#define FML_EXPORT __declspec(dllimport)
-#else
-#define FML_EXPORT __attribute__((visibility("default")))
-#endif
-
-#endif  // FLUTTER_FML_EXPORT_H_