scripts: fix regression in quick_start.py

```
$ ./electrum/scripts/quick_start.py
Traceback (most recent call last):
  File "/home/user/wspace/electrum/./electrum/scripts/quick_start.py", line 32, in <module>
    wallet.start_network(network)
  File "/home/user/wspace/electrum/electrum/wallet.py", line 620, in start_network
    assert self.network is None, "already started"
AssertionError: already started
```
just the sanity check added in 6ac3f84095

electrum/scripts/quick_start.py

@@ -29,7 +29,6 @@ if not os.path.exists(wallet_path):
 
 # open wallet
 wallet = daemon.load_wallet(wallet_path, password=None, upgrade=True)
-wallet.start_network(network)
 
 # you can use ~CLI commands by accessing command_runner
 command_runner = Commands(config=config, daemon=daemon, network=network)