OPENDJ-2521 - Unable to import data into new backend/baseDN using control panel.
With pluggable backends, the import code no longer support the --append Mode (and the replaceExisting option).
ImportLDIF.java: Remove the append and replaceExisting options
ImportTask.java: Remove the append and replaceExisting options and checks done with these options
ControlPanel/NewBaseDNPanel: Remove call to --append option
ControlPanel/ImportLDIFPanel: Remove code and UI elements that deal with the append and replaceExisting options
TestImportAndExport.java: Remove use of append and replaceExisting options
admin_tool*.properties: Remove the labels removed from the Control Panel UI.