Remove `cleaning-interval` remnants.

Since 2008, the cleaning-interval timer has been documented as "effectively obsolete" and disabled in the default configuration with a comment saying "now meaningless".

This change deletes all the code that implements the cleaning-interval timer, except for the config parser in which it is now explicitly marked as deprecated.

I have verified (using the deletelru and deletettl cache stats) that named still cleans the cache after this change.