crypto: fcrypt - Remove support for FCrypt block cipher

crypto: fcrypt - Remove support for FCrypt block cipher

Remove the insecure FCrypt block cipher from the crypto API.  Its only
user was net/rxrpc/, but now net/rxrpc/ implements it locally.  The
crypto API implementation is no longer needed.

For some additional context: FCrypt was designed in 1988 and is
essentially a weakened version of DES.  It has the same 56-bit key size
as DES, which is easily brute forced.  Moreover, it's cryptographically
weak and doesn't even provide the intended 56-bit security level.  Its
author considers it to be a mistake, as well
(https://lists.openafs.org/pipermail/openafs-devel/2000-December/005320.html).

But fortunately this 1980s-era homebrew block cipher was never adopted
outside of net/rxrpc/.  So its code can just be kept there.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> # m68k
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
Link: https://patch.msgid.link/20260522050740.84561-5-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>