drm/bochs: Remove leftover drm_atomic_helper_shutdown

Small mistake that crept into

commit 81da8c3b
Author: Gerd Hoffmann <kraxel@redhat.com>
Date:   Tue Feb 11 14:52:18 2020 +0100

    drm/bochs: add drm_driver.release callback

where drm_atomic_helper_shutdown was left in both places. The
->release callback really shouldn't touch hardware.

Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-28-daniel.vetter@ffwll.ch