cyrpto/b128ops: Remove struct u128

Per git-grep u128_xor() and its related struct u128 are unused except
to implement {be,le}128_xor(). Remove them to free up the namespace.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20230531132323.314826687@infradead.org