Remove the Security Levels icon from the toolbar

Since Security Levels are not something users are expected to change regularly and they come with a lot of caveat and are quite nebulous (See: https://gitlab.torproject.org/tpo/applications/tor-browser/-/issues/42572), I'd like to suggest we remove the icon from the toolbar by default.

The icon could still be re-added by dragging it from the Customize toolbar menu.

What do you think about it?