Remove the login callsign in aprs_network

It's been confusing for a while that when we configured aprsd,
we had to enter the callsign in the [DEFAULT] section and
the [aprs_network] section.

This patch removes the login from the aprs_network section.  aprsd
will now use the main callsign in the [DEFAULT] section as the callsign
to login to the aprsis network.

tests/plugins/test_notify.py

@@ -65,7 +65,6 @@ class TestWatchListPlugin(test_plugin.TestPlugin):
         watchlist_callsigns=DEFAULT_WATCHLIST_CALLSIGNS,
     ):
         CONF.callsign = self.fromcall
-        CONF.aprs_network.login = self.fromcall
         CONF.aprs_fi.apiKey = 'something'
         # Add mock password
         CONF.aprs_network.password = '12345'