The Manage Devices on your account has changed recently, where there is no option to remove devices manually by clicking on the bin icon.
Have a read of this :-
You can have up to6 devices stored on your account and watch on1 device at a time (3if you have NOW Boost).
If you're a BT customer, you can watch on2devices at once as long as one is a BT TV Box and you start streaming in the correct order. For more details, see theStreaming and devices section here. If you have Boost, you can watch on 3 devices at a time.
Once you’ve added devices up to your maximum of 6, when you start watching on a new device the one with the oldestLast used date and no downloads on it will automatically be removed to make space for it. Devices with downloads on them will be the last to be removed.