Use await even more in the "SaveDocument" worker-thread handler

Given that the function is already asynchronous we can make use of `await` even more and reduce the amount of indentation a little bit.

src/core/worker.js

@@ -588,8 +588,8 @@ class WorkerMessageHandler {
             );
           }
         }
+        const refs = await Promise.all(promises);
 
-        return Promise.all(promises).then(refs => {
         let newRefs = [];
         let xfaData = null;
         if (isPureXfa) {
@@ -671,7 +671,6 @@ class WorkerMessageHandler {
         }).finally(() => {
           xref.resetNewTemporaryRef();
         });
-        });
       }
     );