Leave default new mail notification intact

[vanowm]

FireTray is highjacking new mail notification weather Enable mail notification option is enabled or not.
It forcibly disables mail.biff.show_tray_icon preference on startup.
With FireTray it's impossible on Windows system hide tray icon (tray icon available only via try fly-out menu) and show tray icon when new mail received.
With mail.biff.show_tray_icon enabled we can hide main Thunderbird tray icon and always display new mail icon.