drm/vgem: Drop struct drm_vgem_gem_object

Commit 45d9c8dd ("drm/vgem: use shmem helpers") introduced shmem
helpers to vgem and with that, removed all uses of the struct
drm_vgem_gem_object. So, as the struct is no longer used, delete it.

Signed-off-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Melissa Wen <mwen@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230222160617.171429-1-mcanal@igalia.com