Remove Issues::CreateFromVulnerabilityService code

Why are we doing this work

We are changing the workflow for issue creation from vulnerabilities. This issue is for removing code that is no longer needed after changing the workflow. This must be done after #284376 (closed) has been completed. The work for this was previously done in !46503 (diffs), but that MR needed to be broken up due to its complexity.

The following code should be removed:

• ee/app/services/issues/create_from_vulnerability_service.rb: entire file
• ee/spec/services/ee/issues/create_from_vulnerability_service_spec.rb: entire file
• ee/app/controllers/projects/security/vulnerabilities_controller.rb: create_issue method
• ee/config/routes/project.rb: create_issue route

Relevant links

• Initial Issue (includes implementation plan)
• Route change implementation
• Frontend change implementation
• Implementation in previous (closed) MR

Non-functional requirements

None

Implementation plan

• backend Remove unnecessary code
  • ee/app/services/issues/create_from_vulnerability_service.rb: entire file
  • ee/spec/services/ee/issues/create_from_vulnerability_service_spec.rb: entire file
  • ee/app/controllers/projects/security/vulnerabilities_controller.rb: create_issue method
  • ee/config/routes/project.rb: create_issue route