Remove MIPS target check from CI

MIPS was used as a test platform without 64-bit atomics, but it was
apparently downgraded to tier 3 and is currently broken.

.github/workflows/ci.yml

@@ -25,22 +25,7 @@ jobs:
           toolchain: ${{ matrix.rust }}
 
       - name: Run cargo check
-        run: cargo check --benches
-
-  check-no-atomic-u64:
-    name: Check (no AtomicU64)
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout sources
-        uses: actions/checkout@v3
-
-      - name: Install toolchain
-        uses: dtolnay/rust-toolchain@stable
-        with:
-          targets: mips-unknown-linux-gnu
-
-      - name: Run cargo check
-        run: cargo check --target mips-unknown-linux-gnu --no-default-features
+        run: cargo check
 
   test:
     name: Test suite