[SPARK-6103][Graphx]remove unused class to import in EdgeRDDImpl

Class TaskContext is unused in EdgeRDDImpl, so we need to remove it from import list.

Author: Lianhui Wang <lianhuiwang09@gmail.com>

Closes #4846 from lianhuiwang/SPARK-6103 and squashes the following commits:

31aed64 [Lianhui Wang] remove unused class to import in EdgeRDDImpl