Remove open file limit workaround

This leak in GitPython may have been a significant contributor to
our high open file counts in tests:

e16a0040d0

Now that the fix is in a release, remove our ulimit workaround.

Change-Id: Ib61c0ec67d0d244939ee2da142faf03e791159d5

playbooks/zuul-tox/pre.yaml

@@ -4,13 +4,3 @@
     - run-dstat
     - role: ensure-zookeeper
       zookeeper_use_tls: true
-  tasks:
-    # This is needed to run opendev unit test jobs under bionic/focal,
-    # but may not be necessary in later releases
-    - name: Increase open file limit
-      become: True
-      lineinfile:
-        path: /etc/security/limits.conf
-        line: '* soft nofile 4096'
-    - name: Reset SSH connection
-      meta: reset_connection