Remove feature flag stubs `bootstrap_confirmation_modals` and `sandboxed_mermaid`

Allison Brownerequested to merge
ab-remove-bootstrap_confirmation_modals into master

What does this MR do and why?

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

Edited by Allison Browne

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