Remove unchecked spawn functions (#44)
Remove unchecked spawn functions (#44)

This PR removes two niche functions:

* `builder_spawn_unchecked`
* `spawn_unchecked`

Judging by GitHub searches, the first one seems to be completely unused, while the second one is used in two instances. The [first one](https://github.com/Nyrox/raytracer-rs/blob/52f1f3929556268157004a30c4b7b3fdc847540e/src/raytracer/raytracer.rs) can be easily rewritten using scopes, while the [second one](https://github.com/pedrocr/syncer/blob/221b6f49de9f3d4a9d4d016084002b35ce9bffda/src/backingstore/blobstorage.rs) is more difficult for me to evaluate. @pedrocr Do you think `crossbeam::scope` could replace `spawn_unchecked` in your use case?

My motivation for removing these two functions is just general cleanup - I'd like to remove as much old cruft as possible and try to bring the Crossbeam crates to a more mature state.
crossbeam-rs/crossbeam,crossbeam-rs/crossbeam,aturon/crossbeam,crossbeam-rs/crossbeam,crossbeam-rs/crossbeam