ssl/d1_lib.c: remove g_probable_mtu array

ssl/d1_lib.c: remove g_probable_mtu array

Its last actual user was removed in 1620a2e49c77 "Fix dtls_query_mtu
so that it will always either complete with an mtu that is at least
the minimum or it will fail.", and the only remaining user,
dtls1_link_min_mtu(), just returns its last element unconditionally.
Remove g_probable_mtu and convert dtls1_link_min_mtu into a static const
containing the value.

Complements: 1620a2e49c77 "Fix dtls_query_mtu so that it will always either complete with an mtu that is at least the minimum or it will fail."
Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.foundation>
Reviewed-by: Frederik Wedel-Heinen <fwh.openssl@gmail.com>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
MergeDate: Mon May 18 11:09:52 2026
(Merged from https://github.com/openssl/openssl/pull/31175)