drm/i915/gvt: Remove unused intel_vgpu_decode_sprite_plane

intel_vgpu_decode_sprite_plane() was added in 2017 by
commit 9f31d106 ("drm/i915/gvt: Add framebuffer decoder support")
but has remained unused.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Zhenyu Wang <zhenyuw.linux@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241222002043.173080-3-linux@treblig.org


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>