When opening, we consider a forward with a kind above the maximum
allowable set of kinds and a forward of kind CTF_K_UNKNOWN to be a
forward to a struct. Whatever CTF version it was that produced
forwards with no associated kind, it predates anything we can read:
remove this wart.
libctf/
* ctf-open.c (init_types): Remove typeless CTF_K_FORWARD
special-casing.