Rollup merge of #153452 - GuillaumeGomez:migrate-diag, r=JonathanBrouwer

Cleanup unused diagnostic emission methods

Part of rust-lang/rust#153099.

To remove `lint_level`, we need to remove all functions calling it. One of them is `TyCtxt::node_span_lint`, so removing it.

r? @JonathanBrouwer

Author: JonathanBrouwer