gnutls: remove manual DH prime bits setting
This sets the DH group to an unnecessarily and insecure small size. By removing this function call GnuTLS will select an appropriate DH group from the priority string which, when configured appropriately, will always be a 2048-bit or larger group.
More generally, this function is considered deprecated within GnuTLS; GnuTLS always sets the appropriate DH group via the priority string.
See the GnuTLS API documentation for additional details: https://www.gnutls.org/manual/html_node/Core-TLS-API.html
Signed-off-by: William Woodruff william@trailofbits.com