Make IDNA dependency non-optional

[crowlKats]

No description provided.

[valenting]

This commit reverts #728 that made the IDNA dependency optional to reduce wasm file size.
However, users of the url crate, that specify default-features = false would suddenly stop parsing unicode domains.
Let's remove this for now, and we can try to add it back as a #[cfg(feature = "disable-idna")]. Not sure if the idna crate not being optional still adds to the wasm file size, or if it gets trimmed at link time.