Remove `cleaning-interval` remnants.

Merged Tony Finch requested to merge fanf/bind9:u/fanf2/cleanup-cleaning-interval into master

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.

Edited by Ondřej Surý

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