Remove legacy experimental jobs

If projects are interested in these jobs, I think it's better they add
them in-repo these days.  Remove from the integrated-gate definition.

Change-Id: Icb0c3a1c07e7158de1edab97c94a5083106f4064

zuul.d/zuul-legacy-project-templates.yaml

@@ -97,13 +97,6 @@
         - tempest-full
         - neutron-grenade:
             branches: ^(?!stable/(newton|ocata)).*$
-    experimental:
-      jobs:
-        - legacy-tempest-dsvm-neutron-pg-full:
-            voting: false
-        - legacy-tempest-dsvm-neutron-full-opensuse-423:
-            voting: false
-            branches: ^(?!stable/(newton|ocata)).*$
 
 - project-template:
     name: lib-forward-testing