Removes the check for custom-template declaration

[lannka]

This sanity check would potentially break dynamically loaded custom-template, for example from an FIE ad.

Like custom-element, we should just rely on validator to do the job.

[zhouyx]

Do we have validator check on template type now?
Without the check, we could end up with a lot of unresolved promise. I still think we should perform the check but not throw error. Or at least timeout the promise.

[lannka]

validator does check it