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`.
Maniphest Tasks: T100687
Differential Revision: https://developer.blender.org/D15833
a50ca6a1cd