This feature works only for batch inspection and disabled in the editor.¹
According to the JetBrains IntelliJ IDEA 2016.3 EAP Makes Unused Code Detection More Flexible blog, it's now possible.
You can find this in Settings > Editor > Inspections > Java > Declaration redundancy > Unused declaration > on the right.
Image may be NSFW.
Clik here to view.
¹ Anna Kozlova, comment in said blog post, January 11, 2017
