Hide certain structure level timer in projector and speaking times card

[Elblinator]

Current behaviour:
If structur level A was assigned once in a LoS, this structure level will always be displayed in the projector and speaking times card even if this structure level is removed from the LoS

Reproduction:

1. activate structure levels
2. assign a structure level
3. put a participant on the LoS (and assign the structure level if needed)
4. The structure level is now represented in the matrix and the projector
5. remove the participant or change the structure level
6. the structure level is still represented

This could also be an sole backend-issue or a client/backend combination

Wanted behaviour:
If a structure level has the default time (aka. time was not changed via edit, overhang or speaking) and there is no participant on the speaking list this structure level should be removed from the projector and matrix

[luisa-beerboom]

Closing because this will be done in the client (see OpenSlides/openslides-client#3227 )