createGroundPolylineGeometry dependencies, Scene cleanup

[kring]

Hopefully fairly uncontroversial stuff here from our Cesium fork:

• Remove dependency of createGroundPolylineGeometry on GroundPolylinePrimitive. It was only needed for initializing terrain heights, which was delegated to ApproximateTerrainHeights anyway. This change should make that web worker smaller and in my webpack configuration, at least, it avoids a dependency cycle that makes the build hang.
• Make Scene more consistent in how it destroys sub-objects. Random, inconsequential, but overall positive change I had kicking around.

I don't these things need to be mentioned in CHANGES.md, but let me know if you disagree.

[cesium-concierge]

Thanks for the pull request @kring!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

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• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[likangning93]

This change should make that web worker smaller and in my webpack configuration, at least, it avoids a dependency cycle that makes the build hang.

@kring is this generally helpful for anyone using webpack? If so, is it worth a mention in CHANGES.md?

[hpinkos]

Should we make this same change to GroundPrimitive.initializeTerrainHeights?

[hpinkos]

That's my only comment, thanks @kring!

[kring]

@kring is this generally helpful for anyone using webpack? If so, is it worth a mention in CHANGES.md?

I'm not sure. The fun thing about webpack is it has a billion configuration options, so other webpack configurations might not have the same problem. I'll add a note though, can't hurt.

[kring]

This is ready for another look.

[hpinkos]

Thanks @kring!