[SPARK-5982] Remove incorrect Local Read Time Metric

This metric is incomplete, because the files are memory mapped, so much of the read from disk occurs later as tasks actually read the file's data.

This should be merged into 1.3, so that we never expose this incorrect metric to users.

CC pwendell ksakellis sryza

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #4749 from kayousterhout/SPARK-5982 and squashes the following commits:

9737b5e [Kay Ousterhout] More fixes
a1eb300 [Kay Ousterhout] Removed one more use of local read time
cf13497 [Kay Ousterhout] [SPARK-5982] Remove incorrectwq Local Read Time Metric