crypto: drbg - remove unnecessary sanity checks

The drbg_make_shadow function contains sanity checks which are not
needed as the function is invoked at times where it is ensured that the
checked-for variables are available.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>