How to make a Mac forget it ever saw an iOS device?
I'm not sure exactly, but I can help you figure out which files changed after you plugged in the device.  Hopefully you're comfortable with Terminal.

Start a timer.  Plug in your new device, treat it like you would any other new device - trust it, let it sync, whatever.

Once that's all done, stop your timer, see how long it took and round up in minutes.  Let's assume it took 5:18.  We're going to round up to 6 minutes.

$ find ~ -type f -cmin -6 &gt; ~/Desktop/files_modified.txt


Now you have a file showing all of the files that changed in the last 6 minutes.  Some of these files are going to be relevant and most aren't: when I ran this experiment I got lots of files changing related to Safari, for example.

Good luck, I hope that helps.