Remove alias in id Apple.

You can remove aliases except for @iCloud.com, @me.com or @mac.com aliases. If you log in below and the alias has a ? mark on the right side, you can't remove it. If there is an X on the right side, you should be able to remove it.

Apple ID - Manage

You can't remove/change @iCloud.com, @me.com or @mac.com aliases.

iCloud - Create or change email aliases