aqm/codel: remove timestamp update from pull

This can be accomplished by adding a SetTimestampDelta, so no need to do it
here.  Furthermore, it does not check to see whether the Timestamp was empty
or not, so it is setting a weird Timestamp on packets that might have just
been passed-through.

Signed-off-by: Derrick Pallas <pallas@meraki.com>
Merge pull request #160 from pallas/remove-timestamp-update-from-codel-pull

aqm/codel: remove timestamp update from pull