Remove old redirects in Workhorse directory

What does this MR do?

Removes the old redirects in /workhorse/doc/ left in place after the Workhorse docs were moved to the more findable / maintainable https://docs.gitlab.com/ee/development/workhorse/ in !84115 (merged) 5 months ago.

Discussion point before merging: I haven't removed the README.md in /workhorse/ (it's outside of /workhorse/doc) because I wasn't sure if I should.

FYI to @sean_carroll and @tlinz.

Related issues

• Related to Documentation review: Workhorse (#354845 - closed) where the move was orchestrated
• Related to Update links to workhorse files (#357694 - closed)

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