[bfss, migration] fix storage migration (closes #2189206)
The storage migration intend to clean the attributes cache but in practice it
was trying to discard attribut edition. As no attribut were edited this lead to
a crash.
# copyright %(year)s %(author)s, all rights reserved.# contact %(author-web-site)s -- mailto:%(author-email)s#%(long-license)s"""%(distname)s specific hooks and operations"""