Add information about removing a data tier for ECK/self-managed

[yetanothertw]

Manage data tiers in self-managed and ECK could be improved with adding instructions on how to remove data tiers in these deployment types, similar to how we describe this task on the Manage data tiers in ECH or ECE page.

For more information, see Stef's suggestion in #6092 (comment)

[github-actions]

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