To remove a player from your friends, go to the player's profile page and tap on the button with 3 dots at the top right corner and select "unfriend". 

You can also do it from your friends list screen, find your the player you wish to unfriend tap on the 3 dots on the right top corner, and tap on "unfriend". 

Note that you will be able to send a friend request again to this player, but this player won't be able to send you a friend request.