In WT-17656 we added checks to the WT_WITH_SCHEMA_LOCK(_NOWAIT) macros that assert that we are not currently in the middle of a role-transition. This was not allowed to happen with synchronous step-downs, as a concurrent user-initiated schema operation could potentially interfere with the process (e.g. BF-42866).

With asynchronous step-down, concurrent user-driven schema operations should now be permitted. Remove the step-down assertion and update the associated python test test_layered_stepup12.py to cover step-down only (the step-up counterpart is tracked in a separate ticket).