Mark TestingEstimatedDescriptorPropagationTime as obsolete.

We stopped looking at this option in 85cf6dcb, back when we
implemented the minimal pieces of prop275.  Since then, we've had
code to validate and adjust this option, and to give it a different
value in testing networks, but the option hasn't actually done
anything.

We can safely mark it as OBSOLETE, since doing so does not make any
old configuration get rejected.

Closes ticket 32807.