This change helps us avoid changing UNLISTED_ICON_SET's content on both
the service and the k8s configuration.

In order to achieve this, I'm assigning an empty string '' (since
os.environ.get requires a default value), and removing that string with a
simple loop.

This change improves readability of settings.py, and removes the double
assignement, with a cleaner overall routine.

UNLISTED_ICON_SETS is now assigned in settings.py using the function
fetch_and_clean_unlisted_sets.