[schema] remove unique constraint on unlimited String attribute SQL Server doesn't allow indexing (and thus unique constraints) on nvarchar(max). Related to #7173702.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances