Revert "test: adjust rbd test case guards to handle new defaults"

Revert "test: adjust rbd test case guards to handle new defaults"

This reverts commit feb2fc02404775bc262677a2d0434faec0348c53 which
appears to have caused us to lose old format coverage in the Python
bindings tests (rbd_python_api_tests_old_format.yaml).

Unset RBD_FEATURES enviroment variable means "old format".  This
shouldn't be mucked with in any way, see require_new_format() and
create_image() methods in particular.

Fixes: https://tracker.ceph.com/issues/61495
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>