Fix a few pyflakes errors in the server_key_resource

synapse/http/server_key_resource.py

@@ -15,15 +15,11 @@
 
 
 from twisted.web.resource import Resource
-from twisted.web.server import NOT_DONE_YET
-from twisted.internet import defer
 from synapse.http.server import respond_with_json_bytes
-from synapse.crypto.keyclient import fetch_server_key
 from syutil.crypto.jsonsign import sign_json
-from syutil.base64util import encode_base64, decode_base64
+from syutil.base64util import encode_base64
 from syutil.jsonutil import encode_canonical_json
 from OpenSSL import crypto
-from nacl.signing import VerifyKey
 import logging