get rid of the dummy crates

[oli-obk]

rust-lang/cargo#4953 suggests that it should be possible to support importing different versions of the same crate.

[shepmaster]

I would recommend against this. Renaming dependencies in the manifest is the "correct" way to import multiple versions of a crate, but it does not work on versions of Rust before 1.31.0 (2018-12-06).

Using a renamed dependency in your library's Cargo.toml will force any user to upgrade to 1.31+ (or they choose to not use that crate, instead).

To me, the point of this legacy crate is to allow older versions of Rust to use multiple versions of serde. This crate could encourage people that already rely on newer Rust versions to use renaming instead of this crate, however.