[wallet] Remove 'account' argument from GetLegacyBalance()

Summary:
GetLegacyBalance() is never called with an account argument.
Remove the argument and helper functions.

Partial backport of Core PR13825
https://github.com/bitcoin/bitcoin/pull/13825/commits/0629de4919d7b8ffdfb09a6f732dc922d9db843b

Depends on D5316

Test Plan:

ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5410