drm/xe/kunit: Drop xe_wa_test_exit

Remove xe_wa_test_exit() as it could crach the KUnit kernel in
case of hitting some asserts in xe_wa_test_init() as test->priv
could not be pointing to expected data.

 |    # xe_wa_gt: ASSERTION FAILED at drivers/gpu/drm/xe/tests/xe_wa_test.c:34
 |    Expected ret == 0, but
 |        ret == -19 (0xffffffffffffffed)
 |Bus error - the host /dev/shm or /tmp mount likely just ran out of space
 |Kernel panic - not syncing: Kernel mode signal 7

Note that there is no need to call drm_kunit_helper_free_device()
since our fake device allocated by drm_kunit_helper_alloc_device()
will be cleaned up automatically.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250829171922.572-7-michal.wajdeczko@intel.com