Yii1 compatibility

removed YII_PATH constant from the framework to be compatible with Yii 1.1 and more flexible in general.

fixes #4501
I am using it in the classmap which one can replace if needed. If the constant is used in the framework it can not be replaced. There is no case where I can think of changing YII_PATH is necessary but we have seen there was one popping up. Not depending on a constant will avoid this kind of problem in general.