Delete traces of "jet" in test data and unimportant code

compiler/testData/codegen/box/classes/kt1976.kt

@@ -4,5 +4,5 @@ class A {
 
 fun box(): String {
     val a = A()
-    return a.f() // does not work: (in runtime) ClassCastException: A cannot be cast to jet.Function0
+    return a.f() // does not work: (in runtime) ClassCastException: A cannot be cast to kotlin.Function0
 }

compiler/testData/codegen/box/typeMapping/kt3976.kt

@@ -1,4 +1,4 @@
-// java.lang.ClassNotFoundException: jet.Nothing
+// java.lang.ClassNotFoundException: kotlin.Nothing
 
 var currentAccountId: Int? by SessionAccessor()
 class SessionAccessor<T> {

compiler/testData/codegen/box/typeMapping/nothing.kt

@@ -1,5 +1,5 @@
 fun box(): String {
-    // jet.Nothing should not be loaded here
+    // kotlin.Nothing should not be loaded here
     val x = "" is Nothing
     return "OK"
 }

compiler/testData/codegen/box/typeMapping/nullableNothing.kt

@@ -1,6 +1,5 @@
-
 fun box(): String {
-    // This used to be problematic because of an attempt to load jet/Nothing class
+    // This used to be problematic because of an attempt to load kotlin/Nothing class
     val x = "" is Nothing?
     return "OK"
 }