drm/debugfs: Remove the drm_driver.debugfs_cleanup callback

Remove the .debugfs_cleanup() callback now that all the users are gone.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170307204924.1002-3-noralf@tronnes.org