Remove AdSense code, we haven't had ads for years

Fixes a large gap between the navbar and the first header, which could
be observed on browsers running without adblocking enabled.

  <shadowm> Why didn't anyone tell me we had a large gap between the navbar and the first H2 then? :p
  <shadowm> That's a rhetorical question, don't bother to answer.
  <shadowm> The answer is obvious: nobody who cares to report site brokenness visits without an adblocker.
  <shadowm> It's... paradoxical, really.

In Dave's words: "kill it with fire".