`constrainCenter` return value should be discardable

[Mark Pospesel]

Intro

All of our constrain methods return either constraints or dictionaries of constraints but these are always discardable (they do not need to be retained because the constraint's parent view retains them).

constrainCenter was supposed to be marked @discardableResult (it was part of the acceptance criteria for the ticket), but it got overlooked.

Task

mark constrainCenter function return as @discardableResult