Remove sett-rs crate

The maintenance cost of the sett Python bindings is relatively high, both in terms of dev and CI time.

The bindings are only used in Portal and stransfer-sync. The latter one should be retired at some point. This means that we could move only the required parts of the sett-rs crate to Portal - see portal#1069 (closed) - and build it there, which is “smaller”, more predicable environment.

This - theoretically - has a dependency on retiring SFTP, and therefore strasnsfer-sync. In practice, it would already be very hard - and breaking - to make a new release of the python bindings to accomodate strasnsfer-sync.

See milestone 1.17 of the BIWG roadmap.