vhost-device-gpu: Remove old virtio_gpu.rs implementation

Remove the monolithic virtio_gpu.rs file, replaced by the new
modular backend architecture (virgl.rs and gfxstream.rs).

The functionality is now split across:
- backend/virgl.rs (virglrenderer backend)
- backend/gfxstream.rs (gfxstream backend)
- backend/common.rs (shared utilities)

Signed-off-by: Dorinda Bassey <[email protected]>

Author: Dorinda Bassey