[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*