Binding V2 maps YANG enumerations to Java enums

Binding V2 maps enumerations to enums, which is a mistake from binding V1. YANG enumerations are really just a fancy name for string sets, as evidenced by 'enum "*"'. We cannot rely on sensible mapping of enum statement arguments and therefore should not be mapping enumerations to enums.

Furthermore YANG 1.1 allows enumerations to be restricted to a subset of a base enumeration. Since Java enums cannot be subclassed, we cannot form a base/derived type relationship such that a derived enum instance is acceptable by a base enum receiver.