finish backporting PR9602 : remove unused priority number in mempool mapDeltas

Summary:
D4778 and D4787 removed all effect of this number.

(in a follow-up diff, I will remove the RPC parameter slot)

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4812