GetLegacyBalance() is never called with an account argument.
Remove the argument and helper functions.
Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/13825 | PR13825]]
https://github.com/bitcoin/bitcoin/pull/13825/commits/0629de4919d7b8ffdfb09a6f732dc922d9db843b
Depends on D5316