dns.js should not use navtiming API
dns.js should not use navtiming API

This sort of reverts 775516d6

The dns plugin's job is to measure the worst case DNS lookup time from
the user's perspective.  NavTiming tells us the current DNS lookup time,
which might be 0 if cached.  These are two different things.

Additionally, the DNS plugin can be configured to test the DNS of any
host, not just the current page.

We remove this code to make sure the DNS plugin does one job and does it
well.

lognormal/boomerang,lognormal/boomerang