Remove VEGA_DEBUG

[markov00]

Remove this docs after the removal of the VEGA_DEBUG functionality.

See elastic/kibana#234477 or ask the Visualization team for more info.

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/visualize/custom-visualizations-with-vega.md

[florent-leborgne]

Hey @markov00, just want to double check the versions from which this functionality is removed to make sure we're updating the docs correctly:

• if removed only from 9.2 and still present in previous versions (or patches), we need to keep the content and instead mark it as removed in 9.2 using specific tagging in the docs (examples in https://elastic.github.io/docs-builder/contribute/cumulative-docs/example-scenarios/#removed)
• if removed from 9.2, 9.1, and 9.0, and no customer is able to use it in any 9.0+ existing deployment, this PR is fine as is

Let me know, and I'll help with this PR if needed, thanks!

[markov00]

hey @florent-leborgne we are removing this in all version 9.x and 8.x.
I will open a PR in Kibana to remove the docs in 8.19 branch

[florent-leborgne]

LGTM, thank you for making the change directly @markov00!