USB/flash `clear`/`save` behavior w.r.t. default changes

[jordens]

Once things have been saved to flash (also implicitly through e.g. clear), then changes in Default are not propagated. This makes the behavior with an empty flash (that has never seen clear) different from the one after clear. Usual expectation is that clear restores behavior to "what it was before/initially", arguably including transparent propagation/activation of changes to Default with app updates.

The proposed alternative is to use remove_item in current clear and https://github.com/quartiq/stabilizer/issues/860

https://github.com/quartiq/stabilizer/pull/857#discussion_r1540132885