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/wast/src/wast.rs

@@ -202,10 +202,6 @@ impl<T> WastContext<T> {
         {
             return Ok(());
         }
-        if cfg!(feature = "lightbeam") {
-            println!("TODO: Check the assert_trap message: {}", expected);
-            return Ok(());
-        }
         bail!("expected '{}', got '{}'", expected, actual)
     }