PrimitiveCollection.removeAll removes guid

[bkuster]

.removeAll broke .contains:

var collection = new Cesium.PrimitiveCollection();
var primitive = new Cesium.Primitive();
collection.add(primitive);
collection.removeAll();
const contained = collection.contains(primitive);
expect(contained).to.be.false // throws

This PR fixes this issue by removing the collection _guid from all its primitves on removeAll

[cesium-concierge]

Thanks for the pull request @bkuster!

• ✔️ 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.

Reviewers, don't forget to make sure that:

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

[hpinkos]

Good catch @bkuster! Thanks for the pull request. I just had that one comment

[hpinkos]

👍 thanks again!