Make sure deleting items or clearing a complete storage works.

Deleting one item would often delete a linked second item, which
made it hard to remove several items at once.
Discard all image scales that are more than a day older than the context.

Do this on the fly when finding any scale that is outdated.

Refs https://dev.plone.org/ticket/13791
Undo change to __delitem__.

It does not really help.
Test the 'clear' method.
Test newer way of cleaning up old items.
autopep8