Remove code for generating QGreenland plugin manifests

[MattF-NSIDC]

We don't want to build these anymore, and we also don't want to needlessly overwrite the existing manifest with one that may be incompatible with the plugin for whatever users do still dare to use it.

Checklist

If an item on this list is done or not needed, check it with [x] or click the checkbox.

• [x] The PR description links to issues that it resolves with closes #{issue_number}
• [x] Config lockfile updated (inv config.export > qgreenland/config/cfg-lock.json)
• [x] Environment lockfile updated if needed (conda-lock)
• [x] Version bumped if needed (bumpversion (major|minor|patch|prerelease|build))
• [x] CHANGELOG.md updated (for user-facing changes)
• [x] Documentation updated if needed
• [x] New unit tests if needed

[MattF-NSIDC]

#795 needs resolved for this PR to merge