Remove codec feature once MSRV is 1.75

[vmx]

At the time of writing this, Rust 1.75 is only a few months old. Hence we decided to introduce the codec feature, so that projects that don't rely on that trait can also use older Rust versions. For all details see #13.

Once this library is OK with requiring a minimal supported Rust version of at least 1.75, the codec feature should be removed again.