Remove inapplicable TODO re: K<->J type substitution

Turns out the two use cases the TODO worried about are already taken care of: `Unit` is replaced with `void` only in contexts where primitives can be used, and `List` and similar only have incompatible extension methods, which are declared by `CollectionsKt` not `List`. This is likely deliberate to enable the Kotlin <-> Java substitution to be implemented simply by kotlinc.

java/kotlin-extractor/src/main/kotlin/KotlinUsesExtractor.kt

@@ -191,10 +191,6 @@ open class KotlinUsesExtractor(
             logger.error("Unexpected access to anonymous class instance")
         }
 
-        // TODO: only substitute in class and function signatures
-        //       because within function bodies we can get things like Unit.INSTANCE
-        //       and List.asIterable (an extension, i.e. static, method)
-        // Map Kotlin class to its equivalent Java class:
         val substituteClass = getJavaEquivalentClass(c)
 
         val extractClass = substituteClass ?: c