Delete kotlin.jvm.internal.KObject

It was supposed to be used in reflection but the benefits do not outweigh the
cost of KT-6951

 #KT-3223 Fixed
 #KT-6951 Fixed

compiler/testData/writeSignature/declarationSiteVariance/SuperClassWithVariance.kt

@@ -4,4 +4,4 @@ class Derived<A, B, C>: Base<A, B, C>()
 
 // class: Derived
 // jvm signature:     Derived
-// generic signature: <A:Ljava/lang/Object;B:Ljava/lang/Object;C:Ljava/lang/Object;>LBase<TA;TB;TC;>;Lkotlin/jvm/internal/KObject;
+// generic signature: <A:Ljava/lang/Object;B:Ljava/lang/Object;C:Ljava/lang/Object;>LBase<TA;TB;TC;>;