Remove redundant kubernetes version pin

It's already specified in the requirements.txt in the
root of the repo, so everyone developing binderhub locally
has probably been using a much newer version anyway.

I've left the `jupyterhub` pin in as it might need to be
co-ordinated with the version of z2jh the chart uses.

Fixes #1494

helm-chart/images/binderhub/requirements.in

@@ -12,9 +12,6 @@ google-cloud-logging==3.*
 #
 jupyterhub==4.*
 
-# https://github.com/kubernetes-client/python
-kubernetes==9.*
-
 # binderhub's dependencies
 #
 # We can't put ".[pycurl]" here directly as when we freeze this into