Remove cargo check for old rustc versions

Firefox has updated its toolchain to something much more recent.

.github/workflows/main.yml

@@ -166,12 +166,6 @@ jobs:
     - run: cargo check --target wasm32-unknown-emscripten -p wasi-common
     - run: cargo check --target armv7-unknown-linux-gnueabihf -p wasi-common
 
-    # Check that codegen and wasm crates typecheck on 1.43.0; this is required
-    # for Firefox.
-    - run: rustup install 1.43.0
-    - run: cargo +1.43.0 check -p cranelift-codegen
-    - run: cargo +1.43.0 check -p cranelift-wasm
-
   fuzz_targets:
     name: Fuzz Targets
     runs-on: ubuntu-latest