Clear deferred object after its resolved, since the next time reload checks the deferred object existence

-Fix calendar d-n-d works only once

phpgwapi/js/framework/fw_browser.js

@@ -254,6 +254,7 @@ var fw_browser =  Class.extend({
 				if(self.loadingDeferred)
 				{
 					self.loadingDeferred.resolve();
+					self.loadingDeferred = null;
 				}
 			}, 1);
 		}