Make it so client code does not tie tether to wifi

The wifi service already handles turning off tethering if
multi-interface is/isn't supported. We don't need to do tie wifi
and tethering stuff together in the UI.

Test: robotests still pass
Bug: 79213401
Merged-In: I699dfe2d7646f248a54faa5a8429176697614cdf
Change-Id: I699dfe2d7646f248a54faa5a8429176697614cdf