[hooks/syncschema] do not crash on double removal of a constraint
Some complicated migrations may entail a double constraint removal.
It is not yet known whether this is a genuine cw bug or the result
of a complicated migration story, but we should nevertheless
not crash.
Rather, emit a CRITICAL warning and proceed.
Closes #3595309.
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/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html