cocoa: Don't include KHR_portability_enumeration by default.

This is only useful if the application knows to add the flag introduced by this
extension, and at that point the application can also include the extension
themselves.

Case in point: SDL_gpu_vulkan was already doing this!

src/video/cocoa/SDL_cocoavulkan.m

@@ -165,7 +165,7 @@ void Cocoa_Vulkan_UnloadLibrary(SDL_VideoDevice *_this)
 char const * const *Cocoa_Vulkan_GetInstanceExtensions(SDL_VideoDevice *_this, Uint32 *count)
 {
     static const char *const extensionsForCocoa[] = {
-        VK_KHR_SURFACE_EXTENSION_NAME, VK_EXT_METAL_SURFACE_EXTENSION_NAME, VK_KHR_PORTABILITY_ENUMERATION_EXTENSION_NAME
+        VK_KHR_SURFACE_EXTENSION_NAME, VK_EXT_METAL_SURFACE_EXTENSION_NAME
     };
     if(count) {
         *count = SDL_arraysize(extensionsForCocoa);