drm: Remove the obsolete driver-sis

Commit 399516ab ("MAINTAINERS: Add a bunch of legacy (UMS) DRM drivers")
marked sis driver obsolete 7 years ago.
And the mesa UMD of this drm driver already in deprecated list
in the link: https://docs.mesa3d.org/systems.html


Silicon Integrated Systems->drivers/gpu/drm/sis

It's time to remove this driver.

Signed-off-by: Cai Huoqing <cai.huoqing@linux.dev>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20221203102502.3185-6-cai.huoqing@linux.dev