Fix bad oauth2client util import (#273)

diff --git a/googleapiclient/channel.py b/googleapiclient/channel.py
index 7d0feb9..a38b4ff 100644
--- a/googleapiclient/channel.py
+++ b/googleapiclient/channel.py
@@ -61,7 +61,6 @@
 import uuid
 
 from googleapiclient import errors
-from oauth2client import util
 import six
 
 # Oauth2client < 3 has the positional helper in 'util', >= 3 has it