browser-state: Sync SessionManager.removeAll()/removeSessions()

[pocmo]

We need to synchronize calls to removeSessions() and removeAll().

I wonder if we can get rid of removeAll() - removing all sessions including custom tabs seems like something we wouldn't ever want? We use it in tests currently only.

┆Issue is synchronized with this Jira Task

[pocmo]

Firefox for Fire TV currently calls removeAll() here:

https://github.com/mozilla-mobile/firefox-tv/blob/7c3e09fc398eb8e5aad89e3dafa7d0e116150fbf/app/src/main/java/org/mozilla/tv/firefox/session/SessionRepo.kt#L158