Drop old neutron-grenade job
Drop old neutron-grenade job

This job is still running python 2.7. As we are dropping py2 support in
Ussuri cycle, lets drop this job now.
There is same job called "grenade-py3" which runs on python 3 already
and this is still used in project's CI.

Change-Id: I24df43f22a1022a689d3d9fb8705c914e22b0c7f

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder