Remove `mozImageSmoothingEnabled` fallback in `CanvasGraphics.endGroup`
Remove `mozImageSmoothingEnabled` fallback in `CanvasGraphics.endGroup`

This was added all the way back in PR 2936, however it's been unnecessary ever since Firefox 51 (released on 2017-01-24); please see the MDN compatibility data:
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/imageSmoothingEnabled#browser_compatibility

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