Remove deprecated scenario image option

Deprecated scenario image options are being removed from tempest,
see [1]. The tempest change makes img_dir obsolete
therefore CONF.scenario.img_file needs to contain the full path to
the image now.
CONF.scenario.img_dir will be removed entirely from tempest in the
near future by [2].

[1] https://review.opendev.org/c/openstack/tempest/+/710996
[2] https://review.opendev.org/c/openstack/tempest/+/754927

Change-Id: Icf7ac92807873d0a5cd8ccaf75033003adb21c0a

config_tempest/main.py

@@ -89,9 +89,6 @@ def load_basic_defaults(conf):
             ("alt_password", "secrete"),
             ("alt_project_name", "alt_demo")
         ],
-        "scenario": [
-            ("img_dir", "etc")
-        ],
         "auth": [
             ("tempest_roles", "_member_"),
             ("admin_username", "admin"),