fix treatment of wsclean dynamic schemas when pol set from command line

[o-smirnov]

When pol starts as e.g. "I" of unset during finalization, but then changes to "IQ" during validation (because it is set from the command-line, for example), and old copy of wsclean's implicit output structure stays in the schemas, and in the substitution namespace. Add code to reset this every time the dynamic schema is regenerated.