Remove release announcements.

These are now in the `racket/racket-lang-org` repository.
Split `meta/images/taking-screenshots` from the repository.

This is now in the `racket/taking-screenshots` repository.
Remove `mkheart.rkt`, which has been moved to the `racket/drracket`
repository.
Move unix installer test to the "racket-test" pkg.
Now that this isn't an actual package, it doesn't need metadata.