remove calls to schedutil_free_respond()

[garlick]

Problem: schedutil_free_respond() is a no-op at this point because the sched.free request expects no response, and leaving it in the code makes it appear that the protocol requires it.

• https://flux-framework.readthedocs.io/projects/flux-rfc/en/latest/spec_27.html#free

• rfc27: eliminate sched.free response rfc#413

[garlick]

This appeared in flux-core 0.61. If we remove these calls then sched will need to require a minimum of that version.