Always clean up manual tx prioritization deltas #6435

This fixes #4818

An alternative fix is to add the transaction prioritization information to CTxMemPoolEntry itself, instead of storing it separately. This would add 128 bits to every transaction in the mempool, which seems undesirable, given that the majority of transactions won't have a manual transaction prioritization delta.

tested ACK

I horked my fork state, so closing this and recreating.