Remove ourself from the target of the shared NSColorPanel.

The plug-in kept observing the color changes even after the `NSColorPanel` was closed.

Reproduce steps:
1. Show the color panel from the "Edit" -> "Backlight" menu.
2. Close it.
3. Show the color panel from an arbitrary color scheme, "Default" for example: "Xcode" -> "Prefrences" -> "Fonts & Colors" -> "Default" -> "Cursor". 
4. The backlight turned black.