Build failure because `env::remove_var` is now unsafe

Version 1.5.0 [bumped](https://github.com/beeender/glrnvim/commit/d27dfe8119e320f0968dff71c385c44185dedf2a) Rust to the 2024 edition. This breaks the two calls to `env::remove_var` at startup, as that function is [now unsafe](https://doc.rust-lang.org/edition-guide/rust-2024/newly-unsafe-functions.html#stdenvset_var-remove_var) as of the 2024 edition.