How to remove a constraint that is already set in Xcode storyboard
Click on a constraint that you want to delete in the Size inspector, it covers with a thin blue border. Even you can select multiple constraints in Size inspector by keep holding command key. The constraint(s) can then be removed/deleted using the backspace.
You can double click on constraint that you want to delete in the Size inspector and it will shows details of constraint in size inspector. Hit Delete button and it will remove/delete the constraint.