Revert "Include tox_extra_args in tox siblings tasks"

This reverts commit 8b1cc73ee3.

Apparently tox.requires adds lines like this to verbose showconfig
output:

    using tox-3.24.4 from /.../tox/__init__.py (pid 2919)

Which is confusing the current parser in the siblings module. Roll
this back while we work on a fix and regression test.

Change-Id: If3b1d48b36a5d32fddfdabd9c0ec1b81dd6453f2

roles/tox/tasks/siblings.yaml

@@ -9,7 +9,6 @@
     {% if tox_envlist is defined and tox_envlist %}
     -e{{ tox_envlist }}
     {% endif %}
-    {{ tox_extra_args }}
   args:
     chdir: "{{ zuul_work_dir }}"
   environment: "{{ tox_environment|combine(tox_constraints_env|default({})) }}"
@@ -29,7 +28,6 @@
     {% if tox_envlist is defined and tox_envlist %}
     -e{{ tox_envlist }}
     {% endif %}
-    {{ tox_extra_args }}
     > {{ _tox_show_config_tempfile.path }}
   args:
     chdir: "{{ zuul_work_dir }}"