[Core] Eliminate quiet clears

Really this is an invalid behavior aimed at performing better, but
there's nothing backing that up and it's causing issues because not all
change hooks are at the top level nor are they required to be.

Xamarin.PropertyEditing/IGroupingList.cs

@@ -28,10 +28,5 @@ namespace Xamarin.PropertyEditing
 		{
 			get;
 		}
-
-		public void QuietClear()
-		{
-			Items.Clear ();
-		}
 	}
 }

Xamarin.PropertyEditing/ObservableLookup.cs

@@ -193,7 +193,7 @@ namespace Xamarin.PropertyEditing
 		{
 			if (key == null) {
 				bool removed = (this.nullGrouping.Count > 0);
-				this.nullGrouping.QuietClear();
+				this.nullGrouping.Clear();
 				if (removed)
 					OnCollectionChanged (new NotifyCollectionChangedEventArgs (NotifyCollectionChangedAction.Remove, this.nullGrouping, this.groupings.Count));
 
@@ -205,7 +205,7 @@ namespace Xamarin.PropertyEditing
 				var g = this.groupings[index];
 				this.groupings.Remove (key);
 				if (ReuseGroups) {
-					g.QuietClear ();
+					g.Clear ();
 					this.oldGroups.Add (key, g);
 				}
 				
@@ -218,12 +218,12 @@ namespace Xamarin.PropertyEditing
 
 		public void Clear ()
 		{
-			this.nullGrouping?.QuietClear();
+			this.nullGrouping?.Clear();
 
 			if (ReuseGroups) {
 				foreach (var g in this.groupings.Values) {
 					this.oldGroups.Add (g.Key, g);
-					g.QuietClear ();
+					g.Clear ();
 				}
 			}