lib/crypto: powerpc/poly1305: Remove BROKEN implementation

The PowerPC implementation of Poly1305 has been marked as BROKEN for
well over a year, since commit bc8169003b41 ("crypto: powerpc/poly1305 -
add depends on BROKEN for now").  So, it's not being built or used.

The justification for marking it BROKEN was that it didn't work in
softirq context.  That still fully applies.

However, it turns out the assembly file actually also contains logic
bugs that make it compute incorrect Poly1305 values in some cases.  This
is shown by it failing the test case test_poly1305_split_update_carry
(if "depends on BROKEN" is removed).  The cause of that appears to be a
lost carry bit in Carry_reduction.  However, there also appears to be a
bug in do_final_update where it incorrectly assumes v6 is fully reduced.

In addition, poly1305_emit_64 contains a secret-dependent branch.

Let's just drop this code.  A fixed version can be reconsidered later.

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://patch.msgid.link/20261029063522.124421-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>

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