Temporarily disable new x64 backend CI. (#2100)

This CI test has been intermittently failing, which is causing issues
with other PRs. We should turn it back off until we can work out why the
intermittent failures are occuring.

.github/workflows/main.yml

@@ -269,45 +269,6 @@ jobs:
       env:
         RUST_BACKTRACE: 1
 
-  # Perform all tests (debug mode) for `wasmtime` with the experimental x64
-  # backend. This runs on the nightly channel of Rust (because of issues with
-  # unifying Cargo features on stable) on Ubuntu.
-  test_x64:
-    name: Test x64 new backend
-    runs-on: ubuntu-latest
-    steps:
-    - uses: actions/checkout@v2
-      with:
-        submodules: true
-    - uses: ./.github/actions/install-rust
-      with:
-        toolchain: nightly-2020-07-27
-    - uses: ./.github/actions/define-llvm-env
-
-    # Install wasm32 targets in order to build various tests throughout the
-    # repo
-    - run: rustup target add wasm32-wasi
-    - run: rustup target add wasm32-unknown-unknown
-
-    # Build and test all features except for lightbeam
-    - run: |
-        cargo \
-            -Zfeatures=all -Zpackage-features \
-            test \
-            --features test-programs/test_programs \
-            --features experimental_x64 \
-            --all \
-            --exclude lightbeam \
-            --exclude peepmatic \
-            --exclude peepmatic-automata \
-            --exclude peepmatic-fuzzing \
-            --exclude peepmatic-macro \
-            --exclude peepmatic-runtime \
-            --exclude peepmatic-test
-      env:
-        RUST_BACKTRACE: 1
-
-
   # Verify that cranelift's code generation is deterministic
   meta_determinist_check:
     name: Meta deterministic check