Restore previous mouse position when leaving the editor freelook mode

- Remove the crosshair as it no longer serves a purpose (the cursor will
  now appear where the user "expects" it to).

This closes https://github.com/godotengine/godot-proposals/issues/1076.

(cherry picked from commit 4a542e0007)

editor/plugins/spatial_editor_plugin.h

@@ -266,8 +266,8 @@ private:
 
 	bool freelook_active;
 	real_t freelook_speed;
+	Vector2 previous_mouse_position;
 
-	TextureRect *crosshair;
 	Label *info_label;
 	Label *cinema_label;
 	Label *locked_label;