Remove the lightbeam backend (#3390)

This commit removes the Lightbeam backend from Wasmtime as per [RFC 14].
This backend hasn't received maintenance in quite some time, and as [RFC
14] indicates this doesn't meet the threshold for keeping the code
in-tree, so this commit removes it.

A fast "baseline" compiler may still be added in the future. The
addition of such a backend should be in line with [RFC 14], though, with
the principles we now have for stable releases of Wasmtime. I'll close
out Lightbeam-related issues once this is merged.

[RFC 14]: https://github.com/bytecodealliance/rfcs/pull/14

crates/fuzzing/src/oracles.rs

@@ -253,7 +253,7 @@ pub fn differential_execution(
     let configs: Vec<_> = match configs.iter().map(|c| c.to_wasmtime_config()).collect() {
         Ok(cs) => cs,
         // If the config is trying to use something that was turned off at
-        // compile time, eg lightbeam, just continue to the next fuzz input.
+        // compile time just continue to the next fuzz input.
         Err(_) => return,
     };