Remove removal-specific docs tasks

What does this MR do and why?

As per the related issue and MR, we are combining the deprecations and removals pages into one, so the tasks related to the old removals page need to be removed:

• MR template
• Script in pipeline job that checked the old removals page.
• Raketask to update the old removals page.
• Docs about the method of updating the old removals page.
• CODEOWNERS entries for all related files.

Related issues

• Related to gitlab-docs#1517 (closed)
• Related to !123176 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.