Remove resource from externalnamenottested that is already implemented

[mbbush]

It looks like when cloudformation StackSetInstance got implemented, someone forgot to remove it from externalnamenottested.go. I'm cleaning that up.

Description of your changes

I have:

• [ ] Read and followed Crossplane's contribution process.
• [ ] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested