Don't use redundant validation in ChangedLabel importer

What does this MR do and why?

We want to import un/labeled events during GitHub project's import even if such events don't have an associated label (it happens when label, that was assigned/unassigned to/from the issue/MR, was removed afterwards). Here we remove the validation of label presence when importing un/labeled events.

It fixes Missing some labeled events for project imported from GitHub.

Screenshots or screen recordings

Before the fix:

missing_some_labeled_events-bug

After the fix:

missing_some_labeled_events-fix

How to set up and validate locally

1. Have a project on GitHub with an un/labeled issue event
2. Remove associated with the event label
3. Import the project to GitLab
4. Ensure the un/labeled issue event is imported

MR acceptance checklist

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