DRBG: unify initialization and cleanup code

The functions drbg_setup() and drbg_cleanup() used to duplicate a lot of
code from RAND_DRBG_new() and RAND_DRBG_free(). This duplication has been
removed, which simplifies drbg_setup() and makes drbg_cleanup() obsolete.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5294)