[CONFIGURATION-486] Removed static defaultExpressionEngine field.

Using static fields this way is thread-hostile. Hierarchical configurations now
use the default expression engine instance if not specified otherwise. The new
defaults mechanism defined by CONFIGURATION-559 makes it possible to define
another default expression engine globally.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1555841 13f79535-47bb-0310-9956-ffa450edef68
[CONFIGURATION-486] Updated changes.xml.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1555842 13f79535-47bb-0310-9956-ffa450edef68