Removed obsolete method and made PropertyConverter package protected.

The PropertyConverter class is now only accessed by DefaultConversionHandler.
For other components, there is no need to call low-level conversion methods
directly.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/trunk@1516583 13f79535-47bb-0310-9956-ffa450edef68