chromium: Remove .py[co] files prior to building.

Thanks to @iElectric for the notification, although I'm not really sure
whether this will fix the following failed Hydra build:

http://hydra.nixos.org/build/17609086/nixlog/1/raw



The reason is that this failure doesn't happen on every build, but let's
see whether it will happen again now.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>