Remove enablePullToDismiss (#44)

Removed `enablePullToDismiss` from `CupertinoModalSheetPage` as it is no
longer in use.

package/lib/src/modal/cupertino.dart

@@ -408,7 +408,6 @@ class CupertinoModalSheetPage<T> extends Page<T> {
     super.arguments,
     super.restorationId,
     this.maintainState = true,
-    this.enablePullToDismiss = true,
     this.barrierDismissible = true,
     this.fullscreenDialog = false,
     this.barrierLabel,
@@ -432,8 +431,6 @@ class CupertinoModalSheetPage<T> extends Page<T> {
 
   final String? barrierLabel;
 
-  final bool enablePullToDismiss;
-
   final Duration transitionDuration;
 
   final Curve transitionCurve;