Auto merge of #116381 - Mark-Simulacrum:drop-wasm32-unknown-emscripten, r=Kobzol

Remove wasm32-unknown-emscripten tests from CI

This builder tested the wasm32-unknown-emscripten target, which is tier 2 (and so not eligible for testing). In the recent beta [promotion](rust-lang/rust#116362 (comment)), 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.

Author: bors