Remove yaml_safe_load_file patch

What does this MR do and why?

We introduced a temporary monkey patch to the YAML type that makes a safe_load_file method accessible to both Ruby 2 and Ruby 3 (it is only part of psych as of the stdlib gem version bundled with MRI 3.0+)

In addition, we added the same patch in !117073 (merged).

In this MR, we're:

• dropping this initializer;
• remove this patch in lib/gitlab/mail_room.rb.

Refers to: #393651 (closed)

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.