Remove explicit setting of read_ahead for DTLS. It never makes sense not to

Remove explicit setting of read_ahead for DTLS. It never makes sense not to
use read_ahead with DTLS because it doesn't work. Therefore read_ahead needs
to be the default.

Reviewed-by: Andy Polyakov <appro@openssl.org>
(cherry picked from commit f4002412518703d07fee321d4c88ee0bbe1694fe)