tests/server/util: remove in6addr_any for recent MinGW

In ancient MinGW versions, in6addr_any was declared as extern, but not
defined. Because of that, 22a0c577 added
definitions for in6addr_any when compiling with MinGW. The bug was fixed in
w32api version 3.6 from 2006, so this workaround is not needed anymore for
recent versions.

This fixes the following MinGW-w64 warnings because the MinGW-w64 version of
IN6ADDR_ANY_INIT has the two additional braces inside the macro:
util.c:59:14: warning: braces around scalar initializer
util.c:59:40: warning: excess elements in scalar initializer

Ref: https://sourceforge.net/p/mingw/mingw-org-wsl/ci/e4803e0da25c57ae1ad0fa75ae2b7182ff7fa339/tree/w32api/ChangeLog
Closes https://github.com/curl/curl/pull/1379