win32: Remove outdated workaround

The function that was missing in 2016 does exist now.

Fixes gcc complaining about a wrong declaration.

gdk/win32/gdkmonitor-win32.c

@@ -144,20 +144,6 @@ typedef LONG
 #define MONITORINFOF_PRIMARY 1
 #endif
 
-/* MinGW-w64 does not have a prototype for function in its headers
- * at the moment of writing.
- */
-#if !defined (HAVE_SETUP_DI_GET_DEVICE_PROPERTY_W)
-BOOL WINAPI SetupDiGetDevicePropertyW (HDEVINFO          DeviceInfoSet,
-                                       PSP_DEVINFO_DATA  DeviceInfoData,
-                                       const DEVPROPKEY *PropertyKey,
-                                       DEVPROPTYPE      *PropertyType,
-                                       PBYTE             PropertyBuffer,
-                                       DWORD             PropertyBufferSize,
-                                       PDWORD            RequiredSize,
-                                       DWORD             Flags);
-#endif
-
 #define G_GUID_FORMAT "%08lX-%04X-%04X-%02X%02X-%02X%02X%02X%02X%02X%02X"
 #define g_format_guid(guid) (guid)->Data1, \
                             (guid)->Data2, \