track_element: Always emit "control-binding-removed" signal.
When setting a new control binding on a track element, the old control binding (if any) is going to be removed. Make sure the "control-binding-removed" signal is emitted in this case. Fixes https://phabricator.freedesktop.org/T7340#95666 Reviewed-by:Thibault Saunier <thibault.saunier@collabora.com> Differential Revision: https://phabricator.freedesktop.org/D1842