SERVER-35824 changed the setFCV downgrade logic to detect index namespaces that are incompatible with 4.0. The setFCV command would fail in the presence of these long index namespaces.

This downgrade check is not necessary because the startup validation logic in a 4.0 server would detect these invalid index namespaces and fail to start up. The user would still be able to fix the invalid namespaces using a 4.2 binary in 4.0 FCV mode.