[stable-only] Remove grenade-heat job

greanade-heat job was defined in heat stable/train which
is now EOL
- https://review.opendev.org/c/openstack/releases/+/871857

and branch does not exist
- https://github.com/openstack/heat/branches/active

Which introduced zuul config error in grenade gate.

Removing this undefined job.

Change-Id: I7e58183ab6b85264b1a0e0e36fbc92c17046f2c8

.zuul.yaml

@@ -4,8 +4,6 @@
     check:
       jobs:
         - grenade
-        - grenade-heat:
-            voting: false
         - grenade-py3
         - grenade-multinode
         - ironic-grenade-dsvm: