Revert "piuparts.py: set scriptsdir to /etc/piuparts/scripts and run this scripts by default."
See #752256 for a discussion why this was reverted and why it would be
useful to have this still...

This reverts commit 73478066a879fdaf5038acf60cbf9fed2432cb23.
Improve documentation about --scriptsdir (also see #752256)