ot::MeshCoP::BorderAgent::ClearEphemeralKey() method

Cancels the ephemeral key in use if any. Can be used to cancel a previously set ephemeral key before it times out. If the Border Agent is not running or there is no ephemeral key in use, calling this function has no effect. If a commissioner is connected using the ephemeral key and is currently active, calling this method does not change its state. In this case the `IsEphemeralKeyActive()` will continue to return `true` until the commissioner disconnects, or the ephemeral key timeout expires. To terminate active commissioner sessions, use the `Disconnect()` function.