Remove cargo-readme checks

[bstrie]

Crate-level documentation wants to live in two places simultaneously: in the README.md file (so that it shows up nicely on Github) and in the crate-level doc comment (so that it shows up nicely on docs.rs). In the old days, the solution was to have the documentation duplicated in both places and use a tool like cargo-readme to make sure they stayed in sync. But since Rust 1.54 it's now possible to declare a crate-level doc comment via #![doc = include_str!("../README.md")], which allows us to remove the duplicated documentation and remove the need to have cargo-readme run in CI.

Previously some of the crates in the Enarx repo were on MSRVs that predated 1.54. However, as part of #28 we're increasing all of our MSRVs to at least 1.56 (in order to leverage the rust-version key in Cargo.toml), so once that's done we can do this for all the crates in the repo that are still using cargo-readme.