SUNRPC: Remove dead code from rpcsec_gss_krb5

SUNRPC: Remove dead code from rpcsec_gss_krb5

With all per-message crypto operations routed through crypto/krb5,
a substantial body of code in rpcsec_gss_krb5 has no remaining
callers. The internal key derivation functions (krb5_derive_key_v2,
krb5_kdf_hmac_sha2, krb5_kdf_feedback_cmac) and the low-level
crypto primitives (krb5_encrypt, gss_krb5_checksum, krb5_cbc_cts_
encrypt/decrypt, krb5_etm_checksum) are unreachable because their
only call sites were the per-enctype function pointers removed in
previous patches. Delete gss_krb5_keys.c entirely and strip the
dead functions from gss_krb5_crypto.c.

The KUnit test suite in gss_krb5_test.c exercised exactly these
internal functions: RFC 3961 n-fold, RFC 3962 key derivation,
RFC 6803 Camellia key derivation, and RFC 8009 AES-SHA2 key
derivation, plus encryption self-tests that drove the now-removed
encrypt routines. The corresponding test coverage is provided by
the crypto/krb5 selftests in crypto/krb5/selftest.c. Remove the
test file, the RPCSEC_GSS_KRB5_KUNIT_TEST Kconfig symbol, the
.kunitconfig, and all VISIBLE_IF_KUNIT / EXPORT_SYMBOL_IF_KUNIT
annotations.

xdr_process_buf() walked xdr_buf segments through a per-segment
callback and existed solely for the crypto routines in
gss_krb5_crypto.c. With that file removed, xdr_process_buf()
has no remaining callers. Its successor, xdr_buf_to_sg(),
populates a scatterlist directly from an xdr_buf byte range
and was introduced earlier in this series.

With every consumer of struct gss_krb5_enctype removed, replace
its remaining uses with the equivalent fields from struct
krb5_enctype (key_len). Remove struct gss_krb5_enctype, the
supported_gss_krb5_enctypes[] table, gss_krb5_lookup_enctype(),
and the gk5e pointer from krb5_ctx.

Assisted-by: Claude:claude-opus-4-6
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Anna Schumaker <anna.schumaker@hammerspace.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>