Remove ansible-version from base-jobs

It's not necessary, and it is pinned to a no-longer-supported
version so it's a config error.

Change-Id: Id86f5d821dd4317ca1523bdaf5aa7875b0126d0e

zuul.d/jobs.yaml

diff --git a/zuul.d/jobs.yaml b/zuul.d/jobs.yaml
index d31c152..ea7d86d 100644
--- a/zuul.d/jobs.yaml
+++ b/zuul.d/jobs.yaml
@@ -63,7 +63,6 @@
 - job:
     name: base-test
     parent: null
-    ansible-version: 2.8
     description: |
       A job to test changes to the base job without disturbing the
       main job in production.  Not for general use.