Currently, test_repair_succeeds_with_unitialized_bm checks whether
repair finishes successfully and the error is properly handled
if batchlog_manager isn't initialized. Error handling depends on
logs, making the test fragile to external conditions and flaky.
Drop the error handling check, successful repair is a sufficient
passing condition.
Fixes: #21167.
Closesscylladb/scylladb#21208