validation: Use *this in CChainState::LoadMempool

Summary:
This concludes backport of core#20750 [17/17]
https://github.com/bitcoin/bitcoin/pull/20750/commits/8c824819c85005ee6c783e9f8fa43ff91716e33d

Depends on D11203

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11204