Remove "copy_database" helper method.

Probably not used, and it gets more and more difficult to support.

"copy_database" without auth doesn't need a helper method, MotorDatabase's "command" method is sufficient. So that's one argument for dropping the helper method.

"copy_database" with auth, however, requires special socket handling. This requirement for special socket handling complicates Motor's plan to support multiple async frameworks (MOTOR-40, MOTOR-41). Additionally, the server has added authentication features to copydb (SERVER-15684) which makes it harder to fully support from the driver.

All evidence indicates that people don't use copy_database from drivers. If they need it, they can use PyMongo; PyMongo 2.8 will have a full copy_database implementation.