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_platform.h

@@ -92,7 +92,6 @@ typedef struct _GLFWmonitorNS
 {
     CGDirectDisplayID   displayID;
     CGDisplayModeRef    previousMode;
-    id                  screen;
 
 } _GLFWmonitorNS;