xfrm: Drop support for HMAC-RIPEMD-160

Drop support for HMAC-RIPEMD-160 from IPsec to reduce the UAPI surface
and simplify future maintenance.  It's almost certainly unused.

RIPEMD-160 received some attention in the early 2000s when SHA-* weren't
quite as well established.  But it never received much adoption outside
of certain niches such as Bitcoin.

It's actually unclear that Linux + IPsec + HMAC-RIPEMD-160 has *ever*
been used, even historically.  When support for it was added in 2003, it
was done so in a "cleanup" commit without any justification [1].  It
didn't actually work until someone happened to fix it 5 years later [2].
That person didn't use or test it either [3].  Finally, also note that
"hmac(rmd160)" is by far the slowest of the algorithms in aalg_list[].

Of course, today IPsec is usually used with an AEAD, such as AES-GCM.
But even for IPsec users still using a dedicated auth algorithm, they
almost certainly aren't using, and shouldn't use, HMAC-RIPEMD-160.

Thus, let's just drop support for it.  Note: no kconfig update is
needed, since CRYPTO_RMD160 wasn't actually being selected anyway.

References:
  [1] linux-history commit d462985fc1941a47
      ("[IPSEC]: Clean up key manager algorithm handling.")
  [2] linux commit a13366c632132bb9
      ("xfrm: xfrm_algo: correct usage of RIPEMD-160")
  [3] https://lore.kernel.org/all/1212340578-15574-1-git-send-email-rueegsegger@swiss-it.ch

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>