Remove the need to specify API specification format

What does this MR do?

Remove the requirement for users to specify the DAST_API_SPECIFICATION_FORMAT when running an API scan. Removal of this field is due to SOAP scanning no longer being supported.

When running an API scan, DAST will always pass 'openapi' as the format to the ZAP scanner.

What are the relevant issue numbers?

gitlab-org/gitlab#10928 (closed)

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