[llvm-readobj] - Make decode_relrs() don't return Expected<>. NFCI.

The `decode_relrs` helper is declared as:

`Expected<std::vector<Elf_Rel>> decode_relrs(Elf_Relr_Range relrs) const;`

it never returns an error though and hence can be simplified to return
a vector.

Differential revision: https://reviews.llvm.org/D85302