provider_core: Remove two TODO 3.0

provider_core: Remove two TODO 3.0

We need to keep the check for prov == NULL in ossl_provider_libctx
but it is not needed in core_get_libctx as there it can happen only when
there is a serious coding error in a third party provider and returning
NULL as libctx would be seriously wrong as that has a special meaning.

The second TODO is valid but not something that is relevant
for 3.0. Change it into a normal comment.

Fixes #14377

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14535)