Fix PostProcessStageTextureCache.clear, which previously didn't clear anything.

[shunter]

Found via static code analysis.

[cesium-concierge]

Thanks for the pull request @shunter!

• ✔️ 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.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

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

[mramato]

Spoke with @bagnell offline, while this looks like a obvious bug and instamerge, he wants to add run some additional tests just to be sure nothing else is affected. (as soon as he frees up from the thing he's working right now)