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

build.rs

@@ -17,11 +17,7 @@ fn main() -> anyhow::Result<()> {
     );
     let mut out = String::new();
 
-    for strategy in &[
-        "Cranelift",
-        #[cfg(feature = "lightbeam")]
-        "Lightbeam",
-    ] {
+    for strategy in &["Cranelift"] {
         writeln!(out, "#[cfg(test)]")?;
         writeln!(out, "#[allow(non_snake_case)]")?;
         writeln!(out, "mod {} {{", strategy)?;
@@ -185,14 +181,6 @@ fn write_testsuite_tests(
 /// Ignore tests that aren't supported yet.
 fn ignore(testsuite: &str, testname: &str, strategy: &str) -> bool {
     match strategy {
-        #[cfg(feature = "lightbeam")]
-        "Lightbeam" => match (testsuite, testname) {
-            ("simd", _) => return true,
-            ("multi_value", _) => return true,
-            ("reference_types", _) => return true,
-            ("bulk_memory_operations", _) => return true,
-            _ => (),
-        },
         "Cranelift" => match (testsuite, testname) {
             // Skip all reference types tests on the old backend. The modern
             // implementation of reference types uses atomic instructions