Re-enable tab unloading for private browsing tabs

The tab unloading feature was disabled in Firefox because it caused forced restarts after updates ( https://bugzilla.mozilla.org/show_bug.cgi?id=1751366 ). We only updates after Tor Browser is restarted by the user, so we may not be affected by this bug and may benefit from the perf improvements when Tor Browser is under memory pressure.

In 12.5 alpha series we should try reverting this patch and enabling tab unloading and see if this breaks anything.