Remove dast_api_scanner feature flag

What does this MR do and why?

This MR removes the dast_api_scanner as described in #379672 (closed).

We need to get this merged in %15.7 hence it's in the draft state.

Screenshots or screen recordings

No visual changes

How to set up and validate locally

The feature can be verified at Project > Security & Configuration > On-demand scans by creating a new site profile with API option selected.

MR acceptance checklist

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