Flutter for Modroid should use vsync

This patch teaches Flutter how to connect to the vsync service on Modroid.
Also, remove our now-redunant copy of vsync.mojom.

Fixes #589

engine/src/flutter/sky/packages/sky_services/BUILD.gn

@@ -28,6 +28,5 @@ dart_pkg("sky_services") {
     "//sky/services/pointer:interfaces",
     "//sky/services/raw_keyboard:interfaces",
     "//sky/services/updater:interfaces",
-    "//sky/services/vsync:interfaces",
   ]
 }