Remove ability to run AJAX spider with API scan

What does this MR do?

This is being removed because it is an unexpected experience for the user that differs from the existing ZAP experience. It also creates a difficult precedent for the normal spider, which will always run unless we explicitly prevent it from running during an API scan or create a new config option to disable it.

What are the relevant issue numbers?

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer