Remove mention of BootstrapVue in Frontend Dependencies docs

What does this MR do and why?

Remove documentation about BootstrapVue

This also removes the block on it as a dependency. Given GitLab UI now vendors BoostrapVue (&13075 (closed)) it no longer appears in our dependency graph, so it's not likely to be added as an explicit dependency any more.

References

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• Vendor BootstrapVue (&13075 - closed)
• Remove BootstrapVue (&15178)

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