Revert "Convert Constants into an interface."

This reverts commit e7f2f43a715d8216f2d6fe78201e9e0adfacb606.

git-svn-id: https://svn.apache.org/repos/asf/logging/log4j/log4j2/trunk@1621040 13f79535-47bb-0310-9956-ffa450edef68