mingw: remove obsolete IPv6-related code

mingw: remove obsolete IPv6-related code

To support IPv6, Git provided fall back functions for Windows versions
that did not support IPv6. However, as Git dropped support for Windows
XP and prior, those functions are not needed anymore.

Remove those fallbacks by reverting fe3b2b7b827c (Enable support for
IPv6 on MinGW, 2009-11-24) and using the functions directly (without
'ipv6_' prefix).

Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>