[dataimport] Drop reference to the 'source' column (closes #4067694).
CW 3.19 drops the multi-source support. This column does not exist in 3.19.
ifnotrql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',ask_confirm=False):rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',ask_confirm=False)