Don't require to override abstract methods in expect class

#KT-16099 Fixed

compiler/testData/multiplatform/explicitActualOnOverrideOfAbstractMethod/jvm.kt

@@ -0,0 +1,7 @@
+actual abstract class Base {
+    actual abstract fun foo()
+}
+
+actual class DerivedImplicit : Base() {
+    override fun foo() {}
+}