Refactor `destruct_maybe` to be in terms of `destruct_union`
Refactor `destruct_maybe` to be in terms of `destruct_union`

Summary: As a follow up to my work in D26746232 (https://github.com/facebook/flow/commit/bd8ee2d865997fead87536caaa0cb2e421b08d29), refactor `destruct_maybe` to be in terms of `destruct_union`, as now they are equivalent.

Reviewed By: panagosg7

Differential Revision: D26928247

fbshipit-source-id: 7144d4b80999b3dce6e74b71c0314732b1c62ea5

facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow