libctf: Rip out CTFv1 support

CTFv1 was obsolete before libctf was imported into FreeBSD, and
ctfconvert/ctfmerge can emit only CTFv2. Make ctf.h a bit easier to
maintain by ripping v1 support out. No functional change intended.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 7db423d69273644d3be2a65a48a67b9e0cfb0147)