You can remove a friend from your Profile screen by following these steps:

• Tap the icon of the friend you want to remove
• Tap "Remove Friend" at the bottom

Once unfriended, both your names will no longer appear on each other's Friend Rankings. 
Please rest assured that the other player will not be notified that they have been removed.

If you wish to become friends again, you can simply send a new friend request.
Even if you unfriend someone by mistake, you can always re-add them at any time.