drm/fb-helper: Remove drm_fb_helper add, add_all and remove connector functions

drm/fb-helper: Remove drm_fb_helper add, add_all and remove connector functions

drm_fb_helper_single_add_all_connectors(),
drm_fb_helper_add_one_connector()
and drm_fb_helper_remove_one_connector() don't keep an array of
connectors anymore and are just dummy. Now we have no callers to these
functions hence remove them.

Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@intel.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305120434.111091-7-pankaj.laxminarayan.bharadiya@intel.com