Ignore GetSystemStatsRequest in duplicate RPC detection.

Summary:
The memory profiler generates a lot of repeated calls to runtime.memory_usage()
on prod.  These get marked as duplicate RPCs, even though they are intentional.
More generally, two calls to the runtime API may return different data, and
were likely intentionally duplicated, so they should not get marked as such in
the profiler.

Test Plan:
Load /stats/memory (which makes multiple runtime.memory_usage() calls), check
that they are no longer marked as duplicate RPCs.

Reviewers: alan

Reviewed By: alan

Differential Revision: http://phabricator.khanacademy.org/D10411