Remove the unused "GetIsPureXfa" message handler in the worker (PR 13069 follow-up)

Looking at the API, there's no code which actually sends this message. Most likely it's a left-over from a previous version of PR 13069, since the `isPureXfa` parameter is being included in the "GetDoc" message.

src/core/worker.js

@@ -497,10 +497,6 @@ class WorkerMessageHandler {
       });
     });
 
-    handler.on("GetIsPureXfa", function wphSetupGetIsPureXfa(data) {
-      return pdfManager.ensureDoc("isPureXfa");
-    });
-
     handler.on("GetOutline", function wphSetupGetOutline(data) {
       return pdfManager.ensureCatalog("documentOutline");
     });