As a followup to rB6f4e595e9ba9 and as per @Campbell Barton (campbellbarton) suggestion,
this adds EDBM_selectmode_disable_multi, similar to
EDBM_mesh_deselect_all_multi & EDBM_mesh_deselect_all, so other similar
uses don't need to be done in a loop.
Also, selected_objects isn't a reliable way to handle this case - since
objects can be in edit-mode & not selected.