Removed NSScreen member of OS X monitor.

Apple's documentation of [NSScreen screens] mentions that, "The
(screens) array should not be cached.  Screens can be added, removed,
or dynamically reconfigured at any time."  Because of this, we simply
obtain the screen from a displayID whenever we need it.

Fixes #492.
Closes #493.

src/cocoa_monitor.m

@@ -286,7 +286,6 @@ _GLFWmonitor** _glfwPlatformGetMonitors(int* count)
 
         monitor = _glfwAllocMonitor(name, size.width, size.height);
         monitor->ns.displayID = displays[i];
-        monitor->ns.screen = [screens objectAtIndex:j];
 
         free(name);