Get rid of old Windows specific defines

MINGW64
- uses 0x601 as value for _WIN32_WINNT. No need for us to define
it to that value or even lower values in some places.
This also gets rid of:  warning: "_WIN32_WINNT" redefined
- has 0x0502 for WINVER, so get rid of us setting it to 0x0500 in
gimp-app-test-utils.h. It also seems that the need to use G_OS_WIN32
has disappeared here.
- DIRECTINPUT_VERSION is 0x0800, no need for us to set it to that value.
- AI_ADDRCONFIG was apparently missing from the MINGW headers in the
past, but not anymore.

libgimpwidgets/gimpwidgetsutils.c

@@ -37,10 +37,6 @@
 #endif
 
 #ifdef G_OS_WIN32
-#ifdef _WIN32_WINNT
-#undef _WIN32_WINNT
-#endif
-#define _WIN32_WINNT 0x0600
 #include <windows.h>
 #include <icm.h>
 #endif