Remove confirmModalPromise

[cjcenizal]

Per #9984 (comment) discussion, let's remove confirmModalPromise and just use confirmModal instead.

It seems a bit confusing to have two ways of doing the same thing, only with vey slightly different interfaces. Are there use cases where it's better to use one over the other?

Unless there's a compelling reason to use confirmModalPromise over confirmModal, then I think we're adding unnecessary complexity by keeping both. If there are clear use-cases where confirmModalPromise should be used over confirmModal, then I think we need to change the name to reflect that use-case (instead of simply reflecting an implementation detail).

Since confirmModalPromise only being used in two places, I think we should just create and return the promises locally at each call site, and just use confirmModal instead.

We can also dig deeper and see if the promises are even necessary.