win32/vulkancontext: Remove unused variable

Silences a compiler warning.

gdk/win32/gdkvulkancontext-win32.c

@@ -38,7 +38,6 @@ gdk_win32_vulkan_context_create_surface (GdkVulkanContext *context,
 {
   GdkSurface *window = gdk_draw_context_get_surface (GDK_DRAW_CONTEXT (context));
   GdkDisplay *display = gdk_draw_context_get_display (GDK_DRAW_CONTEXT (context));
-  GdkWin32Surface *win32_surface = GDK_WIN32_SURFACE (window);
   VkWin32SurfaceCreateInfoKHR info;
   VkResult result;