[Event Hubs] remove iothub-specific connection string static method

[chradek]

This seems to no longer be needed given the changes we made to how EventHubClient usesconnection strings.

[ramya-rao-a]

More details:

In v5 of the Event Hubs library, we are removing the feature that allows users to use the IotHub connection directly in order to come up with a better unified solution across languages that uses receiver redirection. Until then, users can use the event hub compatible connection string of an IotHub to create an EventHubClient