gfxstream: don't assert without host

When invoking the build in the guest, we don't need
to define this.

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/codegen/scripts/cerealgenerator.py

@@ -576,7 +576,6 @@ class BumpPool;
             envGetOrDefault("GFXSTREAM_HOST_DECODER_DIR",
                             default_host_abs_decoder_destination)
         self.host_script_destination = envGetOrDefault("GFXSTREAM_SCRIPTS_DIR")
-        assert(self.host_script_destination is not None)
 
         if self.cerealFlags & VK_CEREAL_FLAG_GUEST:
             self.addGuestEncoderModule(