Remove the firefox-view button from UI, even when always-on private-browsing mode is disabled

We used to disable firefoxview button with a preference, but since bugzilla bug 1817794 the preference is obsolete.

The button hasn't re-appeared though. I think it may be hidden in private browsing mode (PBM).

So there might be nothing to do other than removing the browser.tabs.firefox-view preference. But we should probably review it.