R1:7f9a9bc800f7

Make Harbormaster objects destructible

Summary:
Ref T13114. See PHI511. Ref T13072. This makes Buildables, Builds, Targets and Artifacts destructible with `bin/remove destroy`.

This might not be totally exhaustive. In particular:

- File artifacts won't destroy the file. This is sort of okay because file artifacts are currently just a file reference, but probably shouldn't be how things work in the long term.
- `BuildCommand` doesn't get cleaned up, but `BuildMessage` does on `Build`. See T13072 for…