drm/fb-helper: Remove drm_fb_helper_unlink_fbi()

There are no callers of drm_fb_helper_unlink_fbi() left. Remove the
function.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114125106.28347-3-tzimmermann@suse.de