PropertyMaps can no longer be saved or deleted

[labkey-adam]

Rationale

Now that we've introduced WritablePropertyMap and used it in every code path that calls delete() or save(), move those persistence methods from PropertyMap to WritablePropertyMap. Eliminate setLock() and checkLock() since PropertyMap is immutable. Simplify initialization of _modified.

Related Pull Requests

• Use WritablePropertyMap #5905