Auto merge of #4973 - alexcrichton:dont-require-emscripten, r=matklad

Remove test that requires Emscripten

This seems relatively unlikely to regress in the future and otherwise arranging
Cargo to have a rustc that has Emscripten support will become difficult in the
near future in the Rust repo. All in all it seems like the course of action to
take is to delete this test for now.