refactor: Clean up CTxMemPool initializer list

Summary:
Shorten the CTxMemPool initializer list using default initialization
for members that dont depend on the constuctor parameters.

This is a backport of core#20222 [3/3]
https://github.com/bitcoin/bitcoin/pull/20222/commits/f15e780b9e57554c723bc02aa41150ecf3e3a8c9

Depends on D10842

Test Plan:
ninja all check-all

eviewers:

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10843