Cocoa: Update MoltenVK support for LunarG SDK

GLFW now checks for the libvulkan.1.dylib loader instead of what is now
the ICD.  This removes checking for libMoltenVK.dylib to avoid cryptic
errors.  This unfortunately also breaks compatibility with the
standalone MoltenVK SDK.

This also removes support for the static loader library as that is not
present in the LunarG SDK.

Related to #870.

src/vulkan.c

@@ -54,7 +54,7 @@ GLFWbool _glfwInitVulkan(int mode)
 #elif defined(_GLFW_WIN32)
     _glfw.vk.handle = _glfw_dlopen("vulkan-1.dll");
 #elif defined(_GLFW_COCOA)
-    _glfw.vk.handle = _glfw_dlopen("libMoltenVK.dylib");
+    _glfw.vk.handle = _glfw_dlopen("libvulkan.1.dylib");
 #else
     _glfw.vk.handle = _glfw_dlopen("libvulkan.so.1");
 #endif