[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.