Remove is_lint field from Level::Error.

Because it's redundant w.r.t. `Diagnostic::is_lint`, which is present
for every diagnostic level.

`struct_lint_level_impl` was the only place that set the `Error` field
to `true`, and it's also the only place that calls
`Diagnostic::is_lint()` to set the `is_lint` field.

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -86,9 +86,7 @@ fn source_string(file: Lrc<SourceFile>, line: &Line) -> String {
 /// Maps `Diagnostic::Level` to `snippet::AnnotationType`
 fn annotation_type_for_level(level: Level) -> AnnotationType {
     match level {
-        Level::Bug | Level::DelayedBug | Level::Fatal | Level::Error { .. } => {
-            AnnotationType::Error
-        }
+        Level::Bug | Level::DelayedBug | Level::Fatal | Level::Error => AnnotationType::Error,
         Level::Warning(_) => AnnotationType::Warning,
         Level::Note | Level::OnceNote => AnnotationType::Note,
         Level::Help | Level::OnceHelp => AnnotationType::Help,