Add key unbinds to DiscussionKeyboardNavigator

What does this MR do?

Addresses the comment here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30144#note_204050258 by adding key unbinds on component destroy as well as a helpful comment.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content