Remove `showFields` option for unused fields in` apostrophe-pieces-widgets`

Setting `limitByAll` or `limitByTag` as a module level option on an `apostrophe-pieces-widget` does not add that field to the schema, but the `showFields` field still has a select option for them. Additional logic is needed to make sure only fields that exist are select options in `showFields`

No file or test is identified in the issue. Start by locating the apostrophe-pieces-widgets schema code that builds showFields options, then compare it with the fields created when limitByAll or limitByTag are configured. Done means absent schema fields no longer appear as showFields choices, with regression coverage if an existing test area is found.

Written by the indexing model from the issue text.