Remove history from MLContext

When working in an environment such as spark-shell with MLContext, typically a user has references to any needed Script objects. Thus, the MLContext script history feature is not really necessary. In addition, the particular implementation can be confusing since the Script objects in the LinkedHashMap values can point to the same Script object, which can change over time, leading to unexpected behavior. This mutability makes the history not necessarily accurate. Therefore, remove the script history feature.