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

fuzz/fuzz_targets/compile.rs

@@ -7,8 +7,3 @@ use wasmtime_fuzzing::oracles;
 fuzz_target!(|data: &[u8]| {
     oracles::compile(data, Strategy::Cranelift);
 });
-
-#[cfg(feature = "lightbeam")]
-fuzz_target!(|data: &[u8]| {
-    oracles::compile(data, Strategy::Lightbeam);
-});