Some code simplification
Some code simplification

Summary: This is a small simplification of the CSE code. It gets rid of its irritable bowel syndrome (ibs): Once upon a time the partitioning of the values into two different patricia trees want meant as a performance optimization. However, as patricia trees now have the ability to erase entire subtrees by bitmask (`erase_all_matching`), partitioning things further in CSE adds no (performance) value.

Reviewed By: int3, amnn

Differential Revision: D17872442

fbshipit-source-id: fecafee0f7c97a51abe5ad77cc24c3799d91bd54

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