Change unused_labels from allow to warn
Fixes#66324, making the unused_labels lint warn instead of allow by default. I'm told @rust-lang/lang will need to review this, and perhaps will want to do a crater run.
Deleted unused labels from compiler and fixed or allowed
unused labels in tests. This patch removes some gratuitous
unused labels and turns off the warning for unused labels
that are a necessary part of tests. This will permit
setting the `unused_labels` lint to `warn`.
