Make plugin-sanity-check job as voting

This commit makes plugin-sanity-check job as voting. We should be able
to do it if the job is stable enough.

Change-Id: Ieff34e2b24f5f9228932383490a115a35e7b1767
diff --git a/.zuul.yaml b/.zuul.yaml
index cdeefe1..be38318 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -344,7 +344,6 @@
     nodeset: ubuntu-bionic
     vars:
       tox_envlist: plugin-sanity-check
-    voting: false
     timeout: 5000
     irrelevant-files:
       - ^.*\.rst$