Re-remove advice on using double nested hashes

What does this MR do and why?

• 0a5e6e68 reintroduced documentation that suggests using a double nested hash to add data tracking attributes. Using this method has led to bugs in the tracking not working properly refs: https://gitlab.com/gitlab-data/product-analytics/-/issues/158#note_861165025 Hence I am removing this again to avoid future bugs.

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