How to (unhide WhatsApp contact / remove WhatsApp account) in Samsung Galaxy S5?
I ran into the same problem recently on my Moto G running Android 5.1. The solution is rather simple but took some time to figure out.


Send a text to the number from WhatsApp if you do not have a chat opened currently with that number.
Open contact>Edit>Separate.
Now that the contact's info is separated, go to the one containing the phone number (the one that's missing its WhatsApp link) and delete the contact.
Open WhatsApp, the chat with the number will still be listed but wont show any name because its not in your contact list anymore. Open the chat>Add to existing contacts.
DONE! The WhatsApp will have reappeared.