drop covariant specialisation for supertypes

compiler/testData/codegen/box/builtinStubMethods/twoListsInSupertypes.kt

@@ -1,26 +0,0 @@
-trait ListAny : List<Any>
-trait ListString : List<String>
-
-trait AddStringImpl {
-    fun add(s: String) {}
-}
-
-class A : ListAny, ListString, AddStringImpl {
-    override fun size(): Int = 0
-    override fun isEmpty(): Boolean = true
-    override fun contains(o: Any?): Boolean = false
-    override fun iterator(): Iterator<String> = null!!
-    override fun containsAll(c: Collection<Any?>): Boolean = false
-    override fun get(index: Int): String = null!!
-    override fun indexOf(o: Any?): Int = -1
-    override fun lastIndexOf(o: Any?): Int = -1
-    override fun listIterator(): ListIterator<String> = null!!
-    override fun listIterator(index: Int): ListIterator<String> = null!!
-    override fun subList(fromIndex: Int, toIndex: Int): List<String> = null!!
-}
-
-fun box(): String {
-    val a = A() as MutableList<String>
-    a.add("Fail")
-    return "OK"
-}