arti-rpc-client-core: remove 'c_str_macro' dependency

Arti now has a MSRV of rust 1.77 which supports C string literals, so 'c_str_macro' isn't needed.

@nickm I noticed this when reviewing !2522 (merged) so I just figured I'd make an MR. If there's any reason to keep that dependency, feel free to close this, I don't have any opinions here.