That would be a bug. It's supposed to allow you to remove her after you finish her personal quest with Iroka where she's required but it seems like the "unlock" coding broke at some point since the 2.0 release and no one mentioned it until your post. Putting together a fix now and will have it out in a bit.
Fix is now out. Download the 4.2 version, move your saves over, and then head onto the main Hin Su overworld map and she'll be unlocked and removable again.