KEYS: remove support for asym_tpm keys

asym_tpm keys are tied to TPM v1.2, which uses outdated crypto and has
been deprecated in favor of TPM v2.0 for over 7 years.  A very quick
look at this code also immediately found some memory safety bugs
(https://lore.kernel.org/r/20220113235440.90439-2-ebiggers@kernel.org).
Note that this code is reachable by unprivileged users.

According to Jarkko (one of the keyrings subsystem maintainers), this
code has no practical use cases, and he isn't willing to maintain it
(https://lore.kernel.org/r/YfFZPbKkgYJGWu1Q@iki.fi

).

Therefore, let's remove it.

Note that this feature didn't have any documentation or tests, so we
don't need to worry about removing those.

Cc: David Howells <dhowells@redhat.com>
Cc: Denis Kenzior <denkenz@gmail.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>