Remove system hooks TEST endpoint

This MR aims to solve #381572 (closed)

I have removed the endpoint and associated documentation because no one is using that.

• https://gitlab.com/gitlab-org/gitlab/-/blob/fe958633ff2edba506ff601166d7955e544e9c68/lib/api/system_hooks.rb#L106-L117
• https://gitlab.com/gitlab-org/gitlab/-/blob/fe958633ff2edba506ff601166d7955e544e9c68/doc/api/system_hooks.md#L127-L155