InstrumentedHandler: Remove duplicate calls to requests.update(...)

The timer is updated inside updateResponses(). This causes the count of this
timer to be double what it should be, and probably biased the times for the
exponentially decaying reservoirs.

Commit 33113e55cb fixed this same bug for activeRequests.
Merge pull request #445 from evanj/master

InstrumentedHandler: Remove duplicate calls to requests.update(...)