[SPARK-4444] Drop VD type parameter from EdgeRDD

Due to vertex attribute caching, EdgeRDD previously took two type parameters: ED and VD. However, this is an implementation detail that should not be exposed in the interface, so this PR drops the VD type parameter.

This requires removing the `filter` method from the EdgeRDD interface, because it depends on vertex attribute caching.

Author: Ankur Dave <ankurdave@gmail.com>

Closes #3303 from ankurdave/edgerdd-drop-tparam and squashes the following commits:

38dca9b [Ankur Dave] Leave EdgeRDD.fromEdges public
fafeb51 [Ankur Dave] Drop VD type parameter from EdgeRDD