Crash when setting an old clipping plane

The way we handle ownership for clipping planes might be too strict. Doing something like assigning a clipping plane to a tileset, assigning a different clipping plane, and then assigning the original clipping plane causes a crash. It might be better to leave the clipping plane deletion up to the user instead?

[Sandcastle demo](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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)
In the demo uncheck the box and then check it again.

Start by running the linked Sandcastle demo and reproduce the crash by unchecking and rechecking the clipping-plane box. Trace how clipping-plane ownership changes when a tileset receives a new plane and then the original one; done means the sequence no longer crashes and the intended deletion behavior is documented or confirmed.

Written by the indexing model from the issue text.