It’s just Editor Preferences.
Edit:
This is moved to
Preferences -> Editor -> Color Scheme -> Kotlin -> Properties and Variables

Thanks rmirabelle for the comment
For the old plugin, you can go to
Preferences -> Editor -> Color Scheme -> Kotlin
Then find Var (mutable variable, parameter or property) and remove Underscored effect.
I guess they just try to discourage using var by adding the annoying underscored. 😛
