remove mabye_unused annotation for GetWalletBalances

Summary: This was supposed to be removed in D9240, but it seems there was a rebase incident.

Test Plan:
ninja
Check that no warning is shown during compilation.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9951