scan: ignore duplicates that are md component devs

scan: ignore duplicates that are md component devs

md devices using an older superblock version have
superblocks at the end of the md device.  For commands
that skip reading the end of devices during filtering,
the md component devs will be scanned, and will appear
as duplicate PVs to the original md device.  Remove
these md components from the list of unused duplicate
devices, so they are treated as if they had been
ignored during filtering.  This avoids the restrictions
that are placed on using PVs with duplicates.