ruby-macho chokes on changing duplicate RPATHs, so we need to strip the
duplicates before trying to relocate them.
This continues #11405. We need this to unblock
Homebrew/homebrew-core#91224.
While we're here, let's get rid of `HOMEBREW_RELOCATE_RPATHS`. We've
been using it for nearly a year with essentially no problems (barring
`pdnsrec`), so I think it is safe to do unconditionally.