Delete the variable JH_DEV

What does this MR do?

This is one of the follow-ups after MR !6007 (merged).

In the previous MR, the way of including ci jobs was changed, and JH also updated their approach. Thus the variable JH_DEV, which was introduced to identify the build environment of JiHu, is no longer required.

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
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• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Documentation created/updated