libc/db: Remove unused hash functions in hash_func.c

Prune unused code hidden behind 'notdef', bringing us in sync with
the changes in OpenBSD. Despite the __default_hash function pointer
having external linkage, no ABI change is expected since it was
never exported.

Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D55842
Reviewed by: allanjude, des
MFC after: 2 weeks