rustc_span: Revert addition of proc_macro field to ExpnKind::Macro

The flag has a vague meaning and is used for a single diagnostic change that is low benefit and appears only under `-Z macro_backtrace`.

compiler/rustc_middle/src/lint.rs

@@ -387,7 +387,7 @@ pub fn in_external_macro(sess: &Session, span: Span) -> bool {
             false
         }
         ExpnKind::AstPass(_) | ExpnKind::Desugaring(_) => true, // well, it's "external"
-        ExpnKind::Macro { kind: MacroKind::Bang, name: _, proc_macro: _ } => {
+        ExpnKind::Macro(MacroKind::Bang, _) => {
             // Dummy span for the `def_site` means it's an external macro.
             expn_data.def_site.is_dummy() || sess.source_map().is_imported(expn_data.def_site)
         }