Revert "WIP: Add toolchain matrix to job"

This reverts commit 77808b7.

dtolnay/rust-toolchain does not support using a matrix as far as I can
tell. From the PR description it looks like the original author (me)
was testing this through CI, no idea how this patch got merged.

.github/workflows/rust.yml

@@ -105,16 +105,13 @@ jobs:
   WASM:
     name: WASM
     runs-on: ubuntu-latest
-    strategy:
-      matrix:
-        toolchain: [stable, beta, nightly, 1.48.0]
     steps:
       - name: Checkout Crate
         uses: actions/checkout@v3
       - name: Install clang
         run: sudo apt-get install -y clang
       - name: Checkout Toolchain
-        uses: dtolnay/rust-toolchain@${{ matrix.toolchain }}
+        uses: dtolnay/rust-toolchain@stable
       - name: Running WASM tests
         env:
           DO_WASM: true