As part of reconciling the catalog and idents during startup recovery we handle background secondary index builds:
https://github.com/mongodb/mongo/blob/6c9adc9a2d518fa046c7739e043a568f9bee6931/src/mongo/db/storage/storage_engine_impl.cpp#L778-L790

louis.williams@mongodb.com mentioned we no longer use background secondary index builds, so it should be safe to remove this code.