Drop "torbrowser.version" preference

"torbrowser.version" is defined based on __BASE_BROWSER_VERSION_QUOTED__, but is only used in one place: https://gitlab.torproject.org/tpo/applications/tor-browser/-/blob/8614325290175a7253f11501d823db65ab805257/browser/components/abouttor/AboutTorMessage.sys.mjs#L30.

But we could just use the existing "browser.startup.homepage_override.torbrowser.version" instead.