Remove "N309 Python 3.x incompatible construct"

This check was implemented as H231 in Hacking 0.6 or newer
(I0bd89fa05d88de34ca62ce59f427468ede226126),
so the duplicate implementation can be removed.

Change-Id: I003d8611621adc53f1afcfd77691f345397670b0
Merge "Remove "N309 Python 3.x incompatible construct""