(#6017) Purge MCX data before import

Without this patch settings linger in the MCX system because old
settings are not always overwritten by the data import.  This patch
addresses the problem by purging all MCX data prior to the import.
Merge remote-tracking branch 'jeffmccune/ccaviness-6017-mcx-should-overwrite-existing-data'

This closes GH-1796