Remove the remoteFetchTime metric.

This metric is confusing: it adds up all of the time to fetch
shuffle inputs, but fetches often happen in parallel, so
remoteFetchTime can be much longer than the task execution time.

@squito it looks like you added this metric -- do you have a use case for it?

cc @shivaram -- I know you've looked at the shuffle performance a lot so chime in here if this metric has turned out to be useful for you!

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #62 from kayousterhout/remove_fetch_variable and squashes the following commits:

43341eb [Kay Ousterhout] Remote the remoteFetchTime metric.