Fix tor-browser's .mozconfig so that ./mach configure succeeds on Linux by default
Currently --enable-tor-browser-update needs to be removed and --with-tor-browser-version=FOO need to be added to the .mozconfig for the tor-browser project to build. The tor-browser-build build system uses its own per-platform mozconfig when building tor-browser releases, so updating the one in tor-browser will not affect official builds.