This was added a while ago for testing purposes, but was never intended to be used. Remove it until upstream libsecp256k1 decides that verification is stable/ready.
Because it appears as though it's optional for use, there have been several efforts to clean it up and get it back in working order. See #5837 #5838 #5811 for examples.
Until this is actually intended to be used, remove it to remove the temptation.