[backport#10973] Remove use of CCoinsViewMemPool::GetCoin in wallet code

Summary:
b1b2b238928e7be044ad62cf1b222464907ece2c This commit does not change behavior. (Russell Yanofsky)

Depends on D5906

This is a partial backport of Core PR10973

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5927