[omemo] Fix clearing devices

omemo/ui.py

@@ -234,7 +234,7 @@ class OMEMOConfigDialog(GajimPluginConfigDialog):
     def cleardevice_button_clicked_cb(self, button, *args):
         active = self.B.get_object('account_combobox').get_active()
         account = self.account_store[active][0]
-        self.plugin.clear_device_list(account)
+        self.plugin.connections[account].clear_device_list()
         self.update_context_list()
 
     def refresh_button_clicked_cb(self, button, *args):