Fix build failure with gnutls 3.0.3 and above.
Fix build failure with gnutls 3.0.3 and above.

gnutls 3.0.3 removed the deprecated gnutls_transport_set_lowat() API.
As we set it to 0, removing the call entirely is the right way to
do it on gnutls 2.12.0 and above.

gobby/net6,gobby/net6,gobby/net6