python-gunicorn: fix build issue with python2

python-gunicorn: fix build issue with python2

Fix:
    http://autobuild.buildroot.org/results/b2d/b2d43dd3c52cc9f586009a10a40f97b07548322d/build-end.log

The issue arise from the fact that we compile all *.py files, but the
_gaiohttp.py file is using Python3 ("yield from") syntax which causes an
error.

So if we are using Python2, delete this file as it won't ever be imported
anyway as it is checked by gunicorn in the __init__.py file of the worker
module.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>