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: I6a4ce001d6cbb4455386285c0d0a43be592e57d7
diff --git a/.zuul.yaml b/.zuul.yaml
index 641bb85..2ce1085 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -690,8 +690,6 @@
             irrelevant-files: *tempest-irrelevant-files
         - neutron-grenade-multinode:
             irrelevant-files: *tempest-irrelevant-files
-        - neutron-grenade:
-            irrelevant-files: *tempest-irrelevant-files
         - grenade-py3:
             irrelevant-files: *tempest-irrelevant-files
         - devstack-plugin-ceph-tempest:
@@ -730,8 +728,6 @@
             irrelevant-files: *tempest-irrelevant-files
         - tempest-full:
             irrelevant-files: *tempest-irrelevant-files
-        - neutron-grenade:
-            irrelevant-files: *tempest-irrelevant-files
         - grenade-py3:
             irrelevant-files: *tempest-irrelevant-files
         - tempest-ipv6-only: