remove old ThermionFlutterWindow class

thermion_flutter/thermion_flutter_platform_interface/lib/thermion_flutter_window.dart

@@ -1,16 +0,0 @@
-abstract class ThermionFlutterWindow {
-
-  int get width;
-  int get height;
-
-  int get handle;
-
-  ///
-  /// Destroy a texture and clean up the texture cache (if applicable).
-  ///
-  Future destroy();
-
-  Future resize(int width, int height, int left, int top);
-
-  Future markFrameAvailable();
-}