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.
We had to remove `filter` from EdgeRDD and move it exclusively to EdgeRDDImpl because it depends on vertex attribute caching.