To remove a snapshot that was created as part of a Volume Set, use the cmdlet
Remove-PsbSnapshotSet. Combine this cmdlet with Get-PsbSnapshotJobHistory to enumerate the snapshots to find the one(s) to remove. An example is shown below.
Remove-PsbSnapshotSet -FlashArrayAddress $FADNS -FlashArrayCredential $FlashArrayCredential -HistoryId $snaps[0].historyid