This implements the TODO that I left as a comment from #15722.
In order to simplify the handling of deleting an absolute rpath that has
a relative duplicate (or vice-versa, i.e., a relative rpath with an
absolute duplicate), we relocate all rpaths first in one pass and then
delete the duplicates in a separate pass.
We currently rely on some lower-level (but still public) methods from
ruby-macho since the standard `#delete_rpath` method changes the order
in which rpaths are resolved. We can switch back to using
`#delete_rpath` when Homebrew/ruby-macho#555 is merged and released.
4c73942fae