First of several incremental patches tightening up interface.py.

Remove some unneeded imports, a constant and a line of dead code.
Document the current external API interface.py provides.

scripts/servers

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 
-from electrum import Interface, SimpleConfig, set_verbosity
+from electrum import SimpleConfig, set_verbosity
 from electrum.network import DEFAULT_SERVERS, filter_protocol
 import time, Queue
 from collections import defaultdict