There is no way to freely unlink one's account from the Steam login at the moment. You will have to contact the Ankama Support, which has the tools to make this kind of changes.
Browse forums
Ankama Trackers
Steam account issue
By Mugcha#4511 May 01, 2017, 22:42:52
AnkaTracker
Hi everybody first time posting here 
I think you should be able to log on and off in game through steam to be able to change accounts if you want to, my game stuck loging in using my steam account so I can't use my Ankama account which I use to play the game on my phone.
does anybody know how to free you steam account so you can log in the game using your Ankama account ??
First Ankama intervention
Respond to this thread
There is no way to freely unlink one's account from the Steam login at the moment. You will have to contact the Ankama Support, which has the tools to make this kind of changes.
I have the same issue apparently