Minor cleanup of the rsa mp limits code

Reduce RSA_MAX_PRIME_NUM to 5.
Remove no longer used RSA_MIN_PRIME_SIZE.
Make rsa_multip_cap honor RSA_MAX_PRIME_NUM.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4905)