Use "unnecessary variable" inspection in J2K

Unused variables are no more treated as unnecessary
Related to KT-15958

j2k/testData/fileOrElement/assignmentExpression/nullability.kt

@@ -2,8 +2,7 @@ import java.util.HashSet
 
 internal class Foo {
     fun foo(o: HashSet<*>) {
-        val o2 = o
         var foo = 0
-        foo = o2.size
+        foo = o.size
     }
 }