Remove good_path_delayed_bug.

It's only has a single remaining purpose: to ensure that a diagnostic is
printed when `trimmed_def_paths` is used. It's an annoying mechanism:
weak, with odd semantics, badly named, and gets in the way of other
changes.

This commit replaces it with a simpler `must_produce_diag` mechanism,
getting rid of a diagnostic `Level` along the way.

compiler/rustc_errors/src/diagnostic.rs

@@ -237,8 +237,7 @@ impl Diagnostic {
         match self.level {
             Level::Bug | Level::Fatal | Level::Error | Level::DelayedBug => true,
 
-            Level::GoodPathDelayedBug
-            | Level::ForceWarning(_)
+            Level::ForceWarning(_)
             | Level::Warning
             | Level::Note
             | Level::OnceNote