Since we track the outputs owned by the wallet with m_txos, we can now
calculate the balance of the wallet by iterating m_txos and summing up
the amounts of the unspent txos.
As ISMINE_USED is not an actual isminetype that we attach to outputs and
was just passed into `CachedTxGetAvailableCredit` for convenience, we
pull the same determining logic from that function into `GetBalances` in
order to preserve existing behavior.
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