All the user of neutron-grenade from Ussuri gate
onwards has been switched to grenade-py3 job (except
neutron which is running neutron-grenade-multinode).
We can remove the job definition also now and all
stable gate using this job will find the job definition
from neutron stable branches.
Depends-On: https://review.opendev.org/#/c/694234/
Change-Id: I631f0bfbe7e4ee2df89e2eca2060331796e7a0b2
0166a1f286