wallet: Remove unused GetLegacyBalance

Summary:
This is a partial backport of Core PR15596 : https://github.com/bitcoin/bitcoin/pull/15596/commits/fac1a0fe54287d819cd0967ad6c75bbcb49b332b

Depends on D6222

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6223