Print out name of non-dynamic sysctl in sysctl_remove_oid_locked

This will provide a slightly better smoking gun than just stating
"can't remove non-dynamic nodes!" when calling sysctl_ctx_free(9)
and sysctl_remove_{name,oid}(9) with a non-dynamic (likely
static) sysctl.

MFC after: 1 week
Sponsored by: Dell EMC Isilon