wireguardctrl: consider eliminating Client.DeviceByIndex

[mdlayher]

This function is arguably less friendly than Client.DeviceByName. On top of this, there is no concept of an interface index in the userspace interface, so in wguser, it is implemented by calling net.InterfaceByIndex and then Client.DeviceByName immediately after.

Removing this function also means that the method Client.DeviceByName could be shortened to just Client.Device.

[olemd]

I agree, remove it. It simplifies the code, both because of less code and because I don't expect anyone would ever use DeviceByIndex via wireguardctrl.

[mdlayher]

SGTM, I think I'll just do it. Anybody who cares about interface index can do a lookup on their own and then use Device(name string). :)