Now that DIB is python3 only we can remove a hack that made sure
scripts outside the chroot ran with the correct version of python.
This is necessary as python3 does not resolve symbolic links to the
binary like python2.x did, which causes element scripts to fail finding
modules when DIB was run from inside a venv.
This patch does the following:
1. Reverts 9c7b8d17144ae04dcd56cd736a8f05c76ec87cc1 which was the
workaround for mixed python2/3 environments.
2. Updates the scripts to use "python3" instead of "python".
Change-Id: If2402bb02fc8a4778fa9434fa167ea1fafd87c28
8b08d212c3