Optimize scene size ( deleteUnusedDeformers ) removed skinClusters from models imported from usd

**Describe the bug**
When using optimize scene size, maya will remove the skinning from models imported from usd. The deleteUnusedDeformers code flags the skinclusters created by the plugin as unused, my guess is because the inputs are not connected.

**Steps to reproduce**
1 - import a model with rigging using the maya usd plugin
2 - move some joints around and observe the model deforming
3 - choose file->optimize scene size and make sure the "Deformers" option is checked
3 - repeat step 2 and observe the skinning got lost ( the skin clusters got deleted)

**Expected behavior**
I would expect the skinning to remain intact. One suspicion was the unconnected inputs on the skinClusters cause this, but I believe we have found other models ( not from usd ) without those that maintained the skinning after optimize scene size.

Thanks,
Koen