Remove the XML file-based persistence fallback that was introduced in
2004 to migrate from a custom XML file to the configuration area
preference store. After 21 years any user who has launched Eclipse even
once has been migrated.

Removed: readPersistedData_file(), compatibleFileProtocol(),
getPersistenceUrl(), the XML tag constants interface, and the related
PERS_FOLDER, PERS_FILENAME, PERS_ENCODING_VERSION constants.

See vogella/eclipse.platform.ui#9