crypto: remove HKDF library

crypto: remove HKDF library

Remove crypto/hkdf.c, since it's no longer used.  Originally it had two
users, but now both of them just inline the needed HMAC computations
using the HMAC library APIs.  That ends up being better, since it
eliminates all the complexity and performance issues associated with the
crypto_shash abstraction and multi-step HMAC input formatting.

Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Keith Busch <kbusch@kernel.org>