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 will be now used in project's CI.

Depends-On: https://review.opendev.org/#/c/695036/

Change-Id: I5cd8e137a3ae06e49a4351629c5eb207c4e6bf1a

.zuul.yaml

@@ -636,7 +636,7 @@
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
-        - neutron-grenade:
+        - grenade-py3:
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
@@ -678,7 +678,7 @@
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$
-        - neutron-grenade:
+        - grenade-py3:
             irrelevant-files:
               - ^.*\.rst$
               - ^doc/.*$