[hooks/syncschema] avoid altering a dropped table If the constraint's rdef is being removed, CWConstraintDelOp doesn't need to do anything. Otherwise it may try to alter a removed table/column. Closes #4154549

if not rql('CWConstraintType X WHERE X name "RQLUniqueConstraint"',
           ask_confirm=False):
    rql('INSERT CWConstraintType X: X name "RQLUniqueConstraint"',
        ask_confirm=False)