refactor: no mempool arg to GetCoinsCacheSizeState

Summary:

Unnecessary argument since we can make use of this->m_mempool

Partial backport of core#22415:
https://github.com/bitcoin/bitcoin/pull/22415/commits/4abf0779d6594e97222279110c328b75b5f3db7b#

Depends on D12229.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12230