Do not override stage-dir in tempest post

Stage dir used to be /opt/stack but now devstack changed to
use ansible user dir as a stage dir (which is the default
value) so the override is not needed anymore.

Change-Id: Id531271da326413c5a88947d9a8b3ac4da68c47d

playbooks/post-tempest.yaml

@@ -2,7 +2,6 @@
   become: true
   vars:
     logs_root: "{{ devstack_base_dir|default('/opt/stack') }}"
-    stage_dir: "{{ devstack_base_dir|default('/opt/stack') }}"
     test_results_stage_name: 'test_results'
   roles:
     - role: process-test-results