Build pipeline: Removed unused dscBuildVariable tasks

[PlagueHO]

Pull Request (PR) description

This PR removes the dscBuildVariable tasks.

This Pull Request (PR) fixes the following issues

Task list

• [x] Added an entry to the change log under the Unreleased section of the file CHANGELOG.md. Entry should say what was changed and how that affects users (if applicable), and reference the issue being resolved (if applicable).
• [ ] Resource documentation added/updated in README.md.
• [ ] Resource parameter descriptions added/updated in README.md, schema.mof and comment-based help.
• [ ] Comment-based help added/updated.
• [ ] Localization strings added/updated in all localization files as appropriate.
• [ ] Examples appropriately added/updated.
• [ ] Unit tests added/updated. See DSC Community Testing Guidelines.
• [ ] Integration tests added/updated (where possible). See DSC Community Testing Guidelines.
• [ ] New/changed code adheres to DSC Community Style Guidelines.

@johlju - could you review when you have time?

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This change is 

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (main@7dc47bb). Click here to learn what that means. The diff coverage is n/a.

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