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

ci/run-tests.sh

@@ -3,7 +3,6 @@
 cargo test \
     --features "test-programs/test_programs" \
     --workspace \
-    --exclude '*lightbeam*' \
     --exclude 'wasmtime-wasi-*' \
     --exclude 'peepmatic*' \
     --exclude wasi-crypto \