Remove deprecated UpdateLiveRegionEvent (#98615)

packages/flutter/lib/src/semantics/semantics_event.dart

@@ -132,33 +132,3 @@ class TapSemanticEvent extends SemanticsEvent {
   @override
   Map<String, dynamic> getDataMap() => const <String, dynamic>{};
 }
-
-/// An event which triggers a polite announcement of a live region.
-///
-/// This requires that the semantics node has already been marked as a live
-/// region. On Android, TalkBack will make a verbal announcement, as long as
-/// the label of the semantics node has changed since the last live region
-/// update. iOS does not currently support this event.
-///
-/// Deprecated. This message was never implemented, and references to it should
-/// be removed.
-///
-/// See also:
-///
-///  * [SemanticsFlag.isLiveRegion], for a description of live regions.
-///
-@Deprecated(
-  'This event has never been implemented and will be removed in a future version of Flutter. References to it should be removed. '
-  'This feature was deprecated after v1.26.0-18.0.pre.',
-)
-class UpdateLiveRegionEvent extends SemanticsEvent {
-  /// Creates a new [UpdateLiveRegionEvent].
-  @Deprecated(
-    'This event has never been implemented and will be removed in a future version of Flutter. References to it should be removed. '
-    'This feature was deprecated after v1.26.0-18.0.pre.',
-  )
-  const UpdateLiveRegionEvent() : super('updateLiveRegion');
-
-  @override
-  Map<String, dynamic> getDataMap() => const <String, dynamic>{};
-}