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_expand/src/base.rs

@@ -811,16 +811,8 @@ impl SyntaxExtension {
         macro_def_id: Option<DefId>,
         parent_module: Option<DefId>,
     ) -> ExpnData {
-        use SyntaxExtensionKind::*;
-        let proc_macro = match self.kind {
-            // User-defined proc macro
-            Bang(..) | Attr(..) | Derive(..) => true,
-            // Consider everthing else to be not a proc
-            // macro for diagnostic purposes
-            LegacyBang(..) | LegacyAttr(..) | NonMacroAttr { .. } | LegacyDerive(..) => false,
-        };
         ExpnData::new(
-            ExpnKind::Macro { kind: self.macro_kind(), name: descr, proc_macro },
+            ExpnKind::Macro(self.macro_kind(), descr),
             parent,
             call_site,
             self.span,