Simplify Rust conditionals
    
    We can greatly simplify the Rust goop now since:
    
    1. upstream now hard requires Rust, so there's no need to make a
       conditional for `--enable-rust`.
    2. we're not rebasing rpm-ostree in f27, so we can simplify the `>= 28`
       conditionals
    3. el7 is strictly backports only, so we can simplify the `0%{?rhel}`
       conditionals (though we still want to support CentOS 7)
    4. the new recommended way of using Rust in el8 is to just
       `BuildRequires: rust-toolset`, so we can drop all the SCL crud.