Remove check for invalid state.

This state is not actually invalid: it can happen when running an empty batch update of >1 value which causes new, empty batch update to run in a `PropertyChanged` handler triggered from the first batch update.

src/Avalonia.Base/ValueStore.cs

@@ -462,10 +462,6 @@ namespace Avalonia
                                 values.Remove(entry.property);
                             }
                         }
-                        else
-                        {
-                            throw new AvaloniaInternalException("Value could not be found at the end of batch update.");
-                        }
 
                         // If a new batch update was started while ending this one, abort.
                         if (_batchUpdateCount > 0)