[cwvreg] complete cw.web.uicfg cleanup callback (closes #2718217)
1dd655788ece introduced insufficient backward compat support
When cubes directly import uicfg from cubicweb.web (or uihelper) this
also creates a reference in the parent cubicweb.web namespace. This
must be cleaned up as reloaded cubes will take back this reference and
the old versions of its appobjects.
