remove `allow_fail` test flag

[yaahc]

As noted in #46488 (comment) we've discussed stabilizing this feature and came to the conclusion that we likely need better support for tests that are allowed to fail in certain situations such as when not run in CI, but that we're not confident that this feature is the best way to solve the specific problems people have. For now we would like to remove this feature and follow up later with a more thorough RFC discussion using concrete examples.

[name1e5s]

@rustbot claim