txbatcher: be careful when removing local transactions

1. Do not remove local transaction in find_base_tx.

This logic was intended to cleanup claim transactions that are
never broadcast (for example, if the counterparty gets a refund)
(see 1bf1de36cb)

However, this code is too unspecific and may result in fund loss,
because the transaction being removed may contain outgoing payments.
For example, if the electrum server is not responsive, the tx will
be seen as local and deleted. In that case, another payment will
be attempted, thus paying twice.

2. Do not remove tx after try_broadcasting returns False.

The server might be lying to us. We can only remove the local tx
if there is a base_tx, because the next tx we create will try to
spend the same output.

tests/test_txbatcher.py

@@ -222,34 +222,6 @@ class TestTxBatcher(ElectrumTestCase):
         assert new_tx.inputs()[0].prevout == tx.inputs()[0].prevout == txin.prevout
         assert output1 in new_tx.outputs()
 
-    @mock.patch.object(wallet.Abstract_Wallet, 'save_db')
-    async def test_remove_local_base_tx(self, mock_save_db):
-        """
-        The swap claim tx does not get broadcast
-        we test that txbatcher.find_base_tx() removes the local tx
-        """
-        self.maxDiff = None
-        # create wallet
-        wallet = self._create_wallet()
-        # mock is_up_to_date
-        wallet.adb.is_up_to_date = lambda: True
-        # do not broadcast, wait forever
-        async def do_wait(x, y):
-            await asyncio.sleep(100000000)
-        self.network.try_broadcasting = do_wait
-        # add swap data
-        wallet.adb.db.transactions[SWAPDATA.funding_txid] = tx = Transaction(SWAP_FUNDING_TX)
-        wallet.adb.receive_tx_callback(tx, tx_height=1)
-        wallet.txbatcher.add_sweep_input('default', SWAP_SWEEP_INFO)
-        txbatch = wallet.txbatcher.tx_batches.get('default')
-        base_tx = await self._wait_for_base_tx(txbatch)
-        self.assertEqual(base_tx.txid(), '80a8cbc42de74cb48a09644c1e438c8b39144bd3b55c574f21d89d05c85fed34')
-        await wallet.stop()
-        txbatch.batch_inputs.clear()
-        wallet.start_network(self.network)
-        base_tx = await self._wait_for_base_tx(txbatch, should_be_none=True)
-        self.assertEqual(base_tx, None)
-
     async def _wait_for_base_tx(self, txbatch, should_be_none=False):
         async with timeout_after(10):
             while True: