Remove TODO in rsa_ameth.c

Remove TODO in rsa_ameth.c

Fixes #14390

The only caller of this function tests EVP_KEYMGMT_is_a() beforehand
which will fail if the RSA key types do not match. So the test is not
necessary. The assert has been removed when it does the test.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14524)