Don't use site's icon as window icon in Windows when in private browsing mode

When the browser is in site-specific browser mode the app browser icon is set to the website's favicon. This icon presumably is cached somewhere by the operating system, so we should not expose the icon in this fashion when in private browsing mode.

This mode can be accessed when:

• browser.ssb.enabled = true

Relevant ticket:

• https://bugzilla.mozilla.org/show_bug.cgi?id=1602194