Your options (at least some have already been mentioned):
1. delete all partitions on the hard drive while connected to 2nd computer via USB adapter/dock.
2. After boot from Windows 10 USB flash drive, be sure you tell it to wipe all partitions. I know the wording may not be this way on Windows 10, as it hasn't been for me at work with Dell Windows 10 DVDs. The wording there is "Restore from Drive" and then "Wipe all" or something to that extent. Somehow I forget the exact wording.
Either way, once you kick off the Windows installation, you should be all set.