gfxstream: nuke libplatform stub

This was added for Fuchsia support, and now we have a
dedicated Fuchsia subdir.

Reviewed-by: Aaron Ruby <aruby@blackberry.com>
Acked-by: Yonggang Luo <luoyonggang@gmail.com>
Acked-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27246>

src/gfxstream/guest/platform/meson.build

@@ -6,5 +6,4 @@ if with_guest_test
   subdir('kumquat')
 else
   subdir('linux')
-  subdir('stub')
 endif