Remove redundant method: initialize_scanner

What does this MR do and why?

Vulnerabilities::ManuallyCreateService#initialize_scanner repeats the one of the base class CreateServiceBase; the code is slightly different but it seems equivalent.

See https://gitlab.com/gitlab-org/gitlab/-/blob/8574323bcd5d473d3e0a7891806bf9340aa084e4/ee/app/services/vulnerabilities/create_service_base.rb#L91-100