[backport#15288] Remove use of GetCountWithDescendants in wallet code

Summary:
291276f7f40df9fcd62e54c016953705bf0ed04a This commit does not change behavior. (Russell Yanofsky)

Depends on D5819

This is a partial backport of Core PR15288.

Test Plan:

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

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5820