Remove unused plugin crates (#8350)

This PR removes the unused crates for plugin support.

We're currently exploring Wasm-based extensions, and it's unlikely that
we'll be reusing any of this existing work.

Release Notes:

- N/A

crates/languages/src/lib.rs

@@ -25,8 +25,6 @@ mod go;
 mod haskell;
 mod html;
 mod json;
-#[cfg(feature = "plugin_runtime")]
-mod language_plugin;
 mod lua;
 mod nu;
 mod ocaml;