contrib: require version 3.4.14 of gnutls

gnutls 3.4.14 has just been released.
This release contains our DTLS patch, so this patch
set this version as minimum and removes the DTLS patch.
As gnutls is updated, this patch removes also the build force
on nettle, removes deprecated format-security patch and
disable non-suitb curves to let gnutls link with some system
provided nettle (like on Fedora).

Change-Id: I073b7409138310c1d749e5ef5a9ddd3937dc1d4a
Tuleap: #783