Revert "Force ansible 2.9 on infra-prod jobs"

This reverts commit 21c6dc02b5.

Everything appears to be working with Ansible 2.9, which does seem to
sugguest reverting this will result in jobs timing out again.  We will
monitor this, and I76ba278d1ffecbd00886531b4554d7aed21c43df is a
potential fix for this.

Change-Id: Id741d037040bde050abefa4ad7888ea508b484f6

zuul.d/infra-prod.yaml

@@ -30,8 +30,6 @@
       infra_prod_playbook_encrypt_log: true
     nodeset:
       nodes: []
-    # Override to 2.9 to see if timeouts in post-run go away.
-    ansible-version: '2.9'
 
 - job:
     name: infra-prod-bootstrap-bridge