[backport#15288] moved remaining g_mempool references out of wallet.cpp

Summary:
I skipped this commit (https://github.com/bitcoin/bitcoin/pull/15288/commits/cc02c796d3517931acc861b0f9bc50e36e1c95f9) backporting PR15288
because we don't use Core's 'smart fee' functionality but later realized that the references to g_mempool in wallet/wallet.cpp still needed
to be moved to prevent problems with further refactoring. Tried to keep this change as close as possible to the original commit.

Test Plan:

cmake .. -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5944