Drop minimal rust/ library

Drop minimal rust/ library

This was my first experiment with using Rust in this way; I gained
a lot of knowledge from it.  But, we don't really gain
anything from the code as it is today - while it is "bit fiddling"
code, the C code is well tested.

We have a lot of compile-time options, and trimming them will be
helpful.

We've also gotten pushback on hard requiring Rust client side.

Instead, what I'd like to do is hopefully soon create an `ostree-system`
crate that uses the existing `ostree` library and can contain
code drained from the rpm-ostree Rust and used by other projects perhaps.

So the goal here is really more Rust, but we need to focus our
efforts on where it's most valuable.