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

docs/contributing-architecture.md

@@ -120,11 +120,9 @@ their implications in Wasmtime:
 * `wasmtime::Module` - this is the representation of a compiled WebAssembly
   module. At this time Wasmtime always assumes that a wasm module is always
   compiled to native JIT code. `Module` holds the results of said compilation,
-  and currently the Cranelift and Lightbeam modes can be used for compiling. It
-  is a goal of Wasmtime to support other modes of representing modules but those
-  are not implemented today just yet. Additionally the Lightbeam compiler has
-  not received maintenance in a long time, so effectively everyone uses
-  Cranelift.
+  and currently Cranelift can be used for compiling. It is a goal of
+  Wasmtime to support other modes of representing modules but those are not
+  implemented today just yet, only Cranelift is implemented and supported.
 
 * `wasmtime_environ::Module` - this is a descriptor of a wasm module's type and
   structure without holding any actual JIT code. An instance of this type is