[Firefox viewer] Stop using FirefoxCom.requestAsync in the DownloadManager

After the changes in https://bugzilla.mozilla.org/show_bug.cgi?id=1757771, that simplified the MOZCENTRAL downloading code, the `ChromeActions.download`-method will no longer invoke the `sendResponse`-callback.
Hence it should no longer be necessary for the `DownloadManager`, in the MOZCENTRAL viewer, to use `FirefoxCom.requestAsync` since no response is ever provided.[1] For the allocated BlobURLs, they should (hopefully) be released when navigating away from the viewer.

---
[1] Note that that was *already* the case, for one of the previous code-paths in the `ChromeActions.download`-method.

web/firefoxcom.js

@@ -116,13 +116,11 @@ class DownloadManager {
       new Blob([data], { type: contentType })
     );
 
-    FirefoxCom.requestAsync("download", {
+    FirefoxCom.request("download", {
       blobUrl,
       originalUrl: blobUrl,
       filename,
       isAttachment: true,
-    }).then(error => {
-      URL.revokeObjectURL(blobUrl);
     });
   }
 
@@ -161,17 +159,10 @@ class DownloadManager {
   download(blob, url, filename) {
     const blobUrl = URL.createObjectURL(blob);
 
-    FirefoxCom.requestAsync("download", {
+    FirefoxCom.request("download", {
       blobUrl,
       originalUrl: url,
       filename,
-    }).then(error => {
-      if (error) {
-        // If downloading failed in `PdfStreamConverter.jsm` it's very unlikely
-        // that attempting to fallback and re-download would be helpful here.
-        console.error("`ChromeActions.download` failed.");
-      }
-      URL.revokeObjectURL(blobUrl);
     });
   }
 }