HashDedup in multi-channel configuration, could hit roxiemem assert during rowRowBuffer

With multiple channels, roxiemem callback removal may hit an assert if asking to remove a non-existent callback (e.g. if already removed).
Without multiple channels this cannot happen.

Check and avoid removal of previously removed hash dedup roxiemem callbacks.

Although I think roxiemem should not assert here, since it only asserts in this situation if slave row managers.