ethernet: return result when removing interface

This is not used yet, but is consistent with the other
EthernetNetworkFactory APIs and will come in handy soon as logging and
error handling is expanded. Also removing NetworkProvider from the class
documentation as EthernetNetworkFactory does not inherit from it.

This change was suggested in aosp/2059590.

Test: TH
Change-Id: I88f77bd461d50d5df73164560bc5a8395e584d05
1 file changed