[server] remove some pre 3.10 compat
Let's assume nobody will try upgrading an instance from pre-3.10
cubicweb to 3.23+. If they do, they're setting themselves up for a lot
of pain anyway.
usr/lib/python2*/*-packages/cubicweb/dataimport/
usr/lib/python2*/*-packages/cubicweb/server/
usr/lib/python2*/*-packages/cubicweb/hooks/
usr/lib/python2*/*-packages/cubicweb/sobjects/
usr/lib/python2*/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/