[backport#15288] Remove uses of fPruneMode in wallet code

Summary:
cc3836e8f90894432db06d9de6b20eac53d93cbe This commit does not change behavior. (Russell Yanofsky)

Depends on D5831

This is a partial backport of Core PR15288

Test Plan:

cmake .. -GNinja -DENABLE_WERROR=ON -DCMAKE_BUILD_TYPE=Debug -DBUILD_BITCOIN_WALLET=OFF
ninja check-all
cmake .. -GNinja -DENABLE_WERROR=ON -DCMAKE_BUILD_TYPE=Debug
ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5839