Remove SaaS feature definition for Secret push protection

Related issue: #471772 (closed)

What does this MR do and why?

Removes SaaS feature definition for Secret push protection. The feature is now available for SaaS, dedicated as well as self-managed instances.

https://docs.gitlab.com/ee/user/application_security/secret_detection/secret_push_protection/

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A