tests: fix TestLNTransport.test_loop for python 3.12

Semantics for server.wait_closed changed in 3.12, and it was always
a no-op in our usage in prior versions.
see https://github.com/python/cpython/issues/104344

electrum/tests/test_lntransport.py

@@ -95,6 +95,5 @@ class TestLNTransport(ElectrumTestCase):
                     await group.spawn(server_shaked.wait())
             finally:
                 server.close()
-                await server.wait_closed()
 
         await f()