librustc_errors: Remove unused annotation style `OldSchoolNoteText`

I could not find any references to it and the `snippet` module does not
seem to be exported publicly, so I think it can be safely removed.

src/librustc_errors/emitter.rs

@@ -1645,7 +1645,7 @@ impl<'a> WritableDst<'a> {
                 }
             }
             Style::Quotation => {}
-            Style::OldSchoolNoteText | Style::MainHeaderMsg => {
+            Style::MainHeaderMsg => {
                 spec.set_bold(true);
                 if cfg!(windows) {
                     spec.set_intense(true)

src/librustc_errors/snippet.rs

@@ -188,7 +188,6 @@ pub enum Style {
     UnderlineSecondary,
     LabelPrimary,
     LabelSecondary,
-    OldSchoolNoteText,
     NoStyle,
     Level(Level),
     Highlight,