EDK: Remove MOJO_SYSTEM_IMPL_EXPORT, system_impl_export.h, etc.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/1346383004 .
diff --git a/mojo/edk/embedder/scoped_platform_handle.h b/mojo/edk/embedder/scoped_platform_handle.h
index e7b56b3..c886a42 100644
--- a/mojo/edk/embedder/scoped_platform_handle.h
+++ b/mojo/edk/embedder/scoped_platform_handle.h
@@ -6,7 +6,6 @@
 #define MOJO_EDK_EMBEDDER_SCOPED_PLATFORM_HANDLE_H_
 
 #include "mojo/edk/embedder/platform_handle.h"
-#include "mojo/edk/system/system_impl_export.h"
 #include "mojo/public/c/system/macros.h"
 #include "mojo/public/cpp/system/macros.h"
 
@@ -14,7 +13,7 @@
 namespace embedder {
 
 // Scoper for |PlatformHandle|s, which are just file descriptors.
-class MOJO_SYSTEM_IMPL_EXPORT ScopedPlatformHandle {
+class ScopedPlatformHandle {
   MOJO_MOVE_ONLY_TYPE(ScopedPlatformHandle)
 
  public: