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
     }
 }

j2k/testData/fileOrElement/ifStatement/withBlocks.kt

@@ -1,6 +1,5 @@
-if (1 > 0) {
-    val n = 1
-    return n
+return if (1 > 0) {
+    1
 } else {
-    return 0
+    0
 }

j2k/testData/fileOrElement/ifStatement/withoutElse.kt

@@ -1,4 +1,3 @@
 if (1 > 0) {
-    val n = 1
-    return n
+    return 1
 }

j2k/testData/fileOrElement/postProcessing/RedundantTypeCastAndInline.kt

@@ -1,9 +1,8 @@
 internal class C {
     fun foo(o: Any) {
         if (o is String) {
-            val s = o
-            val l = s.length
-            val substring = s.substring(l - 2)
+            val l = o.length
+            val substring = o.substring(l - 2)
         }
     }
 }

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
     }
 }

j2k/testData/fileOrElement/toKotlinClasses/iterableAndIterator2.kt

@@ -10,8 +10,7 @@ internal class Test : Iterable<String> {
     }
 
     fun push(i: Iterator<String>): Iterator<String> {
-        val j = i
-        return j
+        return i
     }
 }
 
@@ -21,7 +20,6 @@ internal class FullTest : Iterable<String> {
     }
 
     fun push(i: Iterator<String>): Iterator<String> {
-        val j = i
-        return j
+        return i
     }
 }

j2k/testData/fileOrElement/toKotlinClasses/iterableAndIterator3.kt

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

j2k/testData/fileOrElement/tryWithResource/WithReturnAtEnd.kt

@@ -5,8 +5,7 @@ class C {
         try {
             ByteArrayInputStream(ByteArray(10)).use { stream ->
                 // reading something
-                val c = stream.read()
-                return c
+                return stream.read()
             }
         } catch (e: IOException) {
             println(e)