Remove unused DBCleanup housekeeping job.
The DBCleanup job was added 4 years ago in 9c0fa01 as a workaround for an issue reported in #5482. However, the job's last run time was never updated, meaning the job only ever ran a single time on any MythTV system since it was added. This seems to indicate that running it as a daily job is unnecessary. Furthermore, DB connection handling has been completely rewritten to use per-thread connection pools, and both MySQL and Qt-MySQL have changed significantly in that time, so the old workaround for an old MySQL issue is no longer required. And MDBManager::PurgeIdleConnections() is now used by MDBManager's push/popConnection() functions and MPoolThread::run(), so the originally-intended function of the patch (cleaning up idle connections when no longer required) is now handled--more responsively--by the DB connection pooling code, itself.