Remove the mempool internal wellington upgrade latch
Summary: Treat as it is always on now that wellington is activated. This pulls several required cleanups, but only the minimal subset is part of this diff: more clean ups will be added as follow ups.
Test Plan:
ninja all check-extended
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Subscribers: PiRK
Differential Revision: https://reviews.bitcoinabc.org/D13937