Represent the presence of a backing field with just a protobuf

extension bit, instead of a more complex annotation scheme.  The
scheme was initially introduced when Konan still used JS serializer
extension, and we only could apply an additional annotation.

runtime/src/main/kotlin/konan/internal/Annotations.kt

@@ -28,10 +28,6 @@ package konan.internal
 //@Retention(AnnotationRetention.SOURCE)
 annotation class ExportForCppRuntime(val name: String = "")
 
-// This one is used internally to mark the presence of a backing field
-// in the absence of IR.
-annotation class HasBackingField
-
 /**
  * This annotation denotes that the element is intrinsic and its usages require special handling in compiler.
  */
@@ -44,4 +40,4 @@ annotation class Intrinsic
  */
 @Target(AnnotationTarget.FUNCTION, AnnotationTarget.CONSTRUCTOR, AnnotationTarget.CLASS)
 @Retention(AnnotationRetention.BINARY)
-annotation class ExportForCompiler
+annotation class ExportForCompiler