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

.github/workflows/main.yml

@@ -131,7 +131,6 @@ jobs:
     # Check some feature combinations of the `wasmtime` crate
     - run: cargo check -p wasmtime --no-default-features
     - run: cargo check -p wasmtime --no-default-features --features wat
-    - run: cargo check -p wasmtime --no-default-features --features lightbeam
     - run: cargo check -p wasmtime --no-default-features --features jitdump
     - run: cargo check -p wasmtime --no-default-features --features vtune
     - run: cargo check -p wasmtime --no-default-features --features cache
@@ -307,7 +306,7 @@ jobs:
         RUST_BACKTRACE: 1
       if: matrix.target == ''
 
-    # Build and test all features except for lightbeam
+    # Build and test all features
     - run: ./ci/run-tests.sh --locked
       env:
         RUST_BACKTRACE: 1
@@ -329,16 +328,6 @@ jobs:
       env:
         RUST_BACKTRACE: 1
 
-    # Build and test lightbeam. Note that
-    # Lightbeam tests fail right now, but we don't want to block on that.
-    - run: cargo build --package lightbeam
-      if: matrix.target != 'aarch64-unknown-linux-gnu' && matrix.target != 's390x-unknown-linux-gnu'
-    - run: cargo test --package lightbeam
-      if: matrix.target != 'aarch64-unknown-linux-gnu' && matrix.target != 's390x-unknown-linux-gnu'
-      continue-on-error: true
-      env:
-        RUST_BACKTRACE: 1
-
   # Perform all tests (debug mode) for `wasmtime` with the old x86 backend.
   test_x86:
     name: Test old x86 backend