Drop MonadZero file, class, and instance

[JordanMartinez]

Description of the change

Fixes #67

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable)

[JordanMartinez]

🏓 @thomashoneyman

This is where the MonadZero class is defined. Once this merges, all other libraries that have such instances will also need those instances removed to compile (e.g. contrib libraries).

[JordanMartinez]

So, should we wait until the end before merging this PR?

[thomashoneyman]

No, I think this is fine to go ahead with. The contrib libraries won't depend on the new version of control until they're also being updated for 0.15, so it's not much of an issue.