MainWindow: remove obsolete variable
MainWindow: remove obsolete variable

This should have been removed in a05799ab2b444195238c6b46dc644221739087e0.

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