crypto: api - Remove redundant #ifdef in crypto_yield()

While looking at CONFIG_PREEMPT dependencies treewide the #ifdef in
crypto_yield() matched.

CONFIG_PREEMPT and CONFIG_PREEMPT_VOLUNTARY are mutually exclusive so the
extra !CONFIG_PREEMPT conditional is redundant.

cond_resched() has only an effect when CONFIG_PREEMPT_VOLUNTARY is set,
otherwise it's a stub which the compiler optimizes out.

Remove the whole conditional.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-crypto@vger.kernel.org
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>