TarsnapAccountDialog: clean up _popup
TarsnapAccountDialog: clean up _popup

This should have been part of 208b254f7999c9b44b3a3a9162d14d3b636d1958.

Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui