[electrum] restore tx notification

Summary:
This was lost in https://github.com/Bitcoin-ABC/ElectrumABC/commit/45fc7d8ad4ee18342d0e2f221138546b2e341be7

Remove indentation levels while we are touching this function.

Test Plan:
Run the application in verbose mode (electrum-abc -v), receive a transaction from another wallet (delta amount needs to be > 0, self transfer are negative because of the fee), and check for the log message [ElectrumWindow/test_wallet.TxUpdateMgr] Notifying GUI 1 tx
I'm not sure how to see the actual notifications on Ubuntu, but I remember that there used to be notifs for new transactions in the past.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14522