rand: remove the ossl_rand_pool_add_additional_data() function.

rand: remove the ossl_rand_pool_add_additional_data() function.

This function isn't called from anywhere and cannot easily be used
by the current RNG infrastructure.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/19493)