Removed unused line in GUI Backup

File selection dialog in GUI backup was using an obsolete
(and possibly never working as intented) setReadOnly method.
Removed it, because it neither works nor is needed.

fixes QubesOS/qubes-issues#5348

qubesmanager/backup_utils.py

@@ -76,7 +76,6 @@ def select_path_button_clicked(dialog, select_file=False, read_only=False):
     try:
         if vm.name == socket.gethostname():
             file_dialog = QtWidgets.QFileDialog()
-            file_dialog.setReadOnly(True)
 
             if select_file:
                 file_dialog_function = file_dialog.getOpenFileName