[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.
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/shared/data usr/share/cubicweb/cubes/shared
debian/tmp/usr/share/cubicweb/cubes/shared/wdoc usr/share/cubicweb/cubes/shared