crypto: rng - Remove unused function __crypto_rng_cast()

This fixes the following warning when building with clang:

crypto/rng.c:35:34: error: unused function '__crypto_rng_cast'
    [-Werror,-Wunused-function]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>