fix(GlToast): Event listener memory leak

What does this MR do?

This makes sure that the root listener that gets added if there's an onComplete callback provided is cleaned up.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
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  • [-] Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.