Since above commit, BKE_id_attributes_active_get would also return
"internal" attributes like ".hide_poly" or ".hide_vert.
As a consequence, a couple of poll functions dont return false anymore
(attribute remove, attribute convert), allowing these operators to
execute, but acting on this "internal" layers is just asking for
trouble.

In the UI, we dont see these attributes, because MESH_UL_attributes
checks is_internal, same thing we do now in
BKE_id_attributes_active_get.