[llvm-readobj] Remove --raw-relr

https://reviews.llvm.org/D47919 dumped RELR relocations as
`R_*_RELATIVE` and added --raw-relr (not in GNU) for testing purposes
(more readable than `llvm-readelf -x .relr.dyn`). The option is obsolete
after `llvm-readelf -r` output gets improved (#89162).

Since --raw-relr never seems to get more adoption. Let's remove it to
avoid some complexity.

Pull Request: https://github.com/llvm/llvm-project/pull/89426