Role ensure-tox: Remove obsoleted comment about tox v3 pin

The actual pin was removed in 13925d1965.
This comment is a leftover that is quite misleading.

Change-Id: I65c1196a7c9dfdf5624280251d7f7ecd9df0c283

roles/ensure-tox/tasks/main.yaml

@@ -25,8 +25,6 @@
       command: '{{ ensure_pip_virtualenv_command }} {{ tox_venv_path }}'
 
     - name: Install tox to local venv
-      # We pin tox to version <4 as v4 is not currently compatible with
-      # zuul's tox siblings processing.
       command: '{{ tox_venv_path }}/bin/pip install tox{{ ensure_tox_version }}'
 
     - name: Export installed tox_executable path