[SPARK-4102] Remove unused ShuffleReader.stop() method.

This method is not implemented by the only subclass
(HashShuffleReader), nor is it ever called. While the
use of Scala's fancy "???" was pretty exciting, the method's
existence can only lead to confusion and it therefore should
be deleted.

mateiz was there a reason for adding this that I'm
missing?

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #2966 from kayousterhout/SPARK-4102 and squashes the following commits:

532c564 [Kay Ousterhout] Added back commented-out method, as per Matei's request
904655e [Kay Ousterhout] [SPARK-4102] Remove unused ShuffleReader.stop() method.
[SPARK-4130][MLlib] Fixing libSVM parser bug with extra whitespace

This simple patch filters out extra whitespace entries.

Author: Joseph E. Gonzalez <joseph.e.gonzalez@gmail.com>
Author: Joey <joseph.e.gonzalez@gmail.com>

Closes #2996 from jegonzal/loadLibSVM and squashes the following commits:

e0227ab [Joey] improving readability
e028e84 [Joseph E. Gonzalez] fixing whitespace bug in loadLibSVMFile when parsing libSVM files