[Xwt] Place dialogs correctly

Automatic placement on mac works correctly now so remove the hack to
replace the window. This should fix placement on both Windows and
Mac so the dialogs are correctly centred.

Fixes https://bugzilla.xamarin.com/show_bug.cgi?id=9012

Xwt.Gtk/Xwt.GtkBackend/MessageService.cs

@@ -72,7 +72,6 @@ namespace Xwt.GtkBackend
 			}
 			dialog.TransientFor = parent;
 			dialog.DestroyWithParent = true;
-			PlaceDialog (dialog, parent);
 			return GtkWorkarounds.RunDialogWithNotification (dialog);
 		}
 		
@@ -96,39 +95,5 @@ namespace Xwt.GtkBackend
 					return w;
 			return RootWindow;
 		}
-		
-		/// <summary>
-		/// Positions a dialog relative to its parent on platforms where default placement is known to be poor.
-		/// </summary>
-		public static void PlaceDialog (Gtk.Window child, Gtk.Window parent)
-		{
-			//HACK: Mac GTK automatic window placement is broken
-			if (Platform.IsMac) {
-				if (parent == null) {
-					parent = GetDefaultParent (child);
-				}
-				if (parent != null) {
-					CenterWindow (child, parent);
-				}
-			}
-		}
-		
-		/// <summary>Centers a window relative to its parent.</summary>
-		static void CenterWindow (Gtk.Window child, Gtk.Window parent)
-		{
-			child.Child.Show ();
-			int w, h, winw, winh, x, y, winx, winy;
-			if (child.Visible)
-				child.GetSize (out w, out h);
-			else {
-				w = child.DefaultSize.Width;
-				h = child.DefaultSize.Height;
-			}
-			parent.GetSize (out winw, out winh);
-			parent.GetPosition (out winx, out winy);
-			x = Math.Max (0, (winw - w) /2) + winx;
-			y = Math.Max (0, (winh - h) /2) + winy;
-			child.Move (x, y);
-		}
 	}
 }