[migration] drop identity_relation if it exists
It has been generated erroneously for a while, then fixed but not properly deleted from old
databases.
Don't use IF EXISTS for sqlserver compat.
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py