[SPARK-6754] Remove unnecessary TaskContextHelper

The TaskContextHelper was originally necessary because TaskContext was written in Java, which does
not have a way to specify that classes are package-private, so TaskContextHelper existed to work
around this. Now that TaskContext has been re-written in Scala, this class is no longer necessary.

rxin can you look at this? It looks like you missed this bit of cleanup when you moved TaskContext from Java to Scala in #4324

cc ScrapCodes and pwendell who added this originally.

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #5402 from kayousterhout/SPARK-6754 and squashes the following commits:

f089800 [Kay Ousterhout] [SPARK-6754] Remove unnecessary TaskContextHelper