cgd(4): Remove recently added dependency on adiantum.

While this dependency is technically correct, it triggers a problem
with module initialization for builtin modules that manifests as an
instant crash on boot for x86 (and likely arm) kernels, and that
problem is not trivial to solve immediately.  See the top of
sys/crypto/aes/aes_impl.c for a summary of the problem and why it's
tricky.

So as a stop-gap measure, we'll remove this dependency for now; we
can reinstate it later once the underlying problem with module
initialization order is resolved.

Reported-by: syzbot+e9b3550af985b6557414@syzkaller.appspotmail.com
(actually first reported, to my knowledge, by pgoyette@, but this
line tells syzkaller that we fixed the problem)

(riastradh)