Reevaluate utility and necessity of New Identity button

While testing changes for persistent mode in #447 (closed) discussion arose around if we should keep the New Identity button in Mullvad Browser long term.

As I understand it the New Identity button was inherited from Tor Browser via the base browser set of changes. For the Tor Browser's version of the New Identity button it keeps the Tor service running, closes the current Tor connection, finds a new Tor circuit to reconnect through, and finally resets the browser's state. In contrast, Mullvad Browser only resets the browser's state which in effect is the same as closing the browser and reopening it. I suppose there's a case to be made for having a button that offers a quick (and a little dirty) browser restart in Mullvad Browser, but I don't think it ends up being any faster (either in clock time or number of user actions) than just closing and reopening the browser.

I think it makes sense for us to remove this button, or at least remove it from the toolbar for new installs and deprecate it over time. Alternatively we could change the functionality of the New Identity button to kick off a full browser restart instead of cleaning up state by hand given that we don't need to keep any background services alive. That however would add to the maintenance burden of keeping Gecko in Mullvad Browser up to date and would necessitate supporting three versions of the New Identity button (one in base browser, one in Mullvad Browser, and one in Tor Browser).

@pierov @ma1 @morgan do y'all have any thoughts on this or more context to add here? I assume we don't have metrics on how often this gets used outside of how much we (TPI and Mullvad) use it?