editor: remove start, end, conf and conf_ts props on paste

[rroohhh]

This does not really work well at the moment, because we assume all nodes to have a start and end, so export is gated on it and the timeline sidebar displays "unknown" as timestamp.

To make this proper, we should probably insert the start and end, so that we fit inbetween our neigbours, but this is a bit tricky to do at this point....

[rroohhh]

It also breaks the worker side export.

We currently specify, that start and end are allowed to be null in our document format, so this is technically just our frontend and our backend code not adhering to the document format, so we could also fix those relying on start / end being always set, but not sure that is the more sensible path, because looking at previous / following nodes in slate sucks.