Fix unused_element.

Author: ryojiro

lib/flutter_datetime_picker.dart

@@ -536,13 +536,11 @@ class _BottomPickerLayout extends SingleChildLayoutDelegate {
   _BottomPickerLayout(
     this.progress,
     this.theme, {
-    this.itemCount,
     this.showTitleActions,
     this.bottomPadding = 0,
   });
 
   final double progress;
-  final int? itemCount;
   final bool? showTitleActions;
   final datetime_picker_theme.DatePickerTheme theme;
   final double bottomPadding;