Colorless RACMO wind vectors

[trey-stafford]

Description

Update RACMO wind vectors style: remove color coding for magnitude

Magnitude is represented by the size of the vector lines. The color mapping was
causing conflicts with other layers (like the raster of annual wind speed). This
problem was discovered in: #749

Checklist

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