Bug 1732168 - Part 1: Remove unused ShutdownPhase::Last variant. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D126632

xpcom/base/ShutdownPhase.h

@@ -24,9 +24,8 @@ enum class ShutdownPhase {
   XPCOMShutdownLoaders,
   XPCOMShutdownFinal,
   CCPostLastCycleCollection,
-  ShutdownPhase_Length,          // never pass this value
-  First = AppShutdownConfirmed,  // for iteration
-  Last = XPCOMShutdownFinal
+  ShutdownPhase_Length,         // never pass this value
+  First = AppShutdownConfirmed  // for iteration
 };
 
 }  // namespace mozilla