CI: Remove useless pre task

This was never necessary because C7 IPv6 CI was not enabled at
the time and later we fixed IPv6 in C7 to avoid this issue.
In Ussuri C7 is going away so even more reasons to drop it. :-)

Change-Id: I4066c9cd86ff892d78f6713589f9afffc611dcc1

tests/pre.yml

@@ -38,13 +38,3 @@
       hostname:
         name: "{{ inventory_hostname }}"
       become: true
-
-    # NOTE(yoctozepto): CentOS 7 image uses myhostname plugin for NSS
-    # which creates issues with IPv6-only deployment by providing
-    # an IPv4 address for the current hostname (affects rabbitmq)
-    - name: Disable myhostname NSS plugin
-      become: true
-      replace:
-        path: /etc/nsswitch.conf
-        regexp: ' myhostname'
-        replace: ''