Data: move element cache to element[expando]

- avoid explicit data.discard() cleanup calls
- explicitly remove the data.events property, only when private data exists
- reduces code footprint

Fixes gh-1734
Close gh-1428

src/event.js

@@ -216,8 +216,12 @@ jQuery.event = {
 
 		// Remove the expando if it's no longer used
 		if ( jQuery.isEmptyObject( events ) ) {
+			// Normally this should go through the data api
+			// but since event.js owns these properties,
+			// this exception is made for the sake of optimizing
+			// the operation.
 			delete elemData.handle;
-			dataPriv.remove( elem, "events" );
+			delete elemData.events;
 		}
 	},