network panel: never show the proxy switch - it doesn't actually do anything
right now, except confuse people.

- Proxy settings are just used, if they exist.  There's no on/off
- TODO:
    - Understand proxy settings better
    - implement switch (mutually-exclusive switches?) correctly
    - add "apply system-wide" button
    - understand how to let i.e. firefox and chrome figure out the
      system proxy settings (for me at least, chrome considers mine
      an unsupported system in its config gui)
    - Initial bit of Upstream implementation of system-wide action here:
      https://github.com/linuxmint/cinnamon-control-center/tree/proxy-system-wide