Remove reading/setting HubAuth SSL attributes in singeuser

Since JupyterHub 1.1.0, the HubAuth SSL attribute values are read
from the same environment variables.

It was introduced by this PR:
jupyterhub/jupyterhub#2588

Author: cmd-ntrf

batchspawner/singleuser.py

@@ -12,9 +12,6 @@
 def main(argv=None):
     port = random_port()
     hub_auth = HubAuth()
-    hub_auth.client_ca = os.environ.get("JUPYTERHUB_SSL_CLIENT_CA", "")
-    hub_auth.certfile = os.environ.get("JUPYTERHUB_SSL_CERTFILE", "")
-    hub_auth.keyfile = os.environ.get("JUPYTERHUB_SSL_KEYFILE", "")
 
     url = url_path_join(hub_auth.api_url, "batchspawner")
     headers = {"Authorization": f"token {hub_auth.api_token}"}