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

README.md

@@ -70,8 +70,8 @@ Hello, world!
   quickly generate high-quality machine code at runtime.
 
 * **Configurable**. Whether you need to precompile your wasm ahead of time,
-  generate code blazingly fast with Lightbeam, or interpret it at runtime,
-  Wasmtime has you covered for all your wasm-executing needs.
+  or interpret it at runtime, Wasmtime has you covered for all your
+  wasm-executing needs.
 
 * **WASI**. Wasmtime supports a rich set of APIs for interacting with the host
   environment through the [WASI standard](https://wasi.dev).