libzstd: Don't clean up files

The cleaning up is already covered by 'nvidia-Makefile'.

This change stops flatpak-builder from inadvertently removing the
'lib' -> 'extra' symlink at the "clean up stage", which was preventing
apps from locating driver libraries.

org.freedesktop.Platform.GL.nvidia.json.in

@@ -28,7 +28,6 @@
             "name": "libzstd",
             "buildsystem": "simple",
             "subdir": "lib",
-            "cleanup": ["*"],
             "build-options": {
                 "cppflags": "-DZSTD_MULTITHREAD"
             },