Safer memory cleanup in (crypto/rsa/rsa_lib.c)

We don't need to use secure clean for public key.

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Paul Yang <yang.yang@baishancloud.com>
(Merged from https://github.com/openssl/openssl/pull/7363)

(cherry picked from commit c033101d)