Phantom root of prefab in prefab stage shouldn't be editable for alternative columns

Steps to reproduce:

1. Create a prefab
2. Enter prefab stage via ">"
3. enable alternative columns such as layer, static 
4. Change layer for the root node

Actual results: user is able to change layer for the grey root node

Expected results: user shouldn't be able to change layer for the grey root node

Reproducible with versions: 6.3&4

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): mac

Notes:

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