Remove .then from FutureSplitter
Remove .then from FutureSplitter

Summary: To allow deprecation, remove use cases of Future::then inside FutureSplitter.h

Reviewed By: yfeldblum

Differential Revision: D9995264

fbshipit-source-id: d44c63c45a08ca675b9bff33fabcbb2836ea2a64

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