Drop the wasm32 builder

This builder tested the wasm32-unknown-emscripten target, which is tier
2 (and so not eligible for testing). In the recent beta promotion, we
ran into a problem with this target: emscripten doesn't support
passing environment variables into the std environment, so we can't
enable RUSTC_BOOTSTRAP for libtest in order to pass -Zunstable-options.

We worked around this for the beta/stable branches, but given this
problem, and its tier 2 status, just dropping the target's tests
entirely seems warranted. Downgrading to tier 3 may also be a good idea,
but that is a separate conversation not proposed here.

.github/workflows/ci.yml

@@ -264,9 +264,6 @@ jobs:
           - name: test-various
             os: ubuntu-20.04-8core-32gb
             env: {}
-          - name: wasm32
-            os: ubuntu-20.04-8core-32gb
-            env: {}
           - name: x86_64-gnu
             os: ubuntu-20.04-4core-16gb
             env: {}