Use "unnecessary variable" inspection in J2K

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

j2k/testData/fileOrElement/toKotlinClasses/iterableAndIterator.kt

@@ -8,8 +8,7 @@ internal class Test : Iterable<String> {
     }
 
     fun push(i: Iterator<String>): Iterator<String> {
-        val j = i
-        return j
+        return i
     }
 }
 
@@ -19,7 +18,6 @@ internal class FullTest : Iterable<String> {
     }
 
     fun push(i: Iterator<String>): Iterator<String> {
-        val j = i
-        return j
+        return i
     }
 }