gnutls: fix CVE-2023-0361 timing side-channel in the TLS RSA key exchange code

Remove branching that depends on secret data.

since the `ok` variable isn't used any more, we can remove all code
used to calculate it

Signed-off-by: Vivek Kumbhar <vkumbhar@mvista.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>