drm: sti: remove vtg_remove

vtg_remove does nothing just remove it

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1510146367-25329-1-git-send-email-benjamin.gaignard@linaro.org