Remove from boneyard after migrating back

Tapping the boneyard warns about this formula, since it's also in the
main repo. It was migrated back in
Homebrew/homebrew@7f06d2facd093330cc51312751c5c83b9aac1c9b. Hence, I
guess we should remove it from the boneyard.

Warning output:

```
$ brew tap homebrew/boneyard
Cloning into '/usr/local/Library/Taps/homebrew-boneyard'...
remote: Reusing existing pack: 565, done.
remote: Total 565 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (565/565), 109.85 KiB | 0 bytes/s, done.
Resolving deltas: 100% (276/276), done.
Checking connectivity... done.
Warning: Could not tap homebrew/boneyard/pulseaudio over Homebrew/homebrew/pulseaudio
Tapped 39 formula
```