[sledge] Remove exclusive-or formula
[sledge] Remove exclusive-or formula

Summary: It is redundant with `Not Iff`

Reviewed By: jvillard

Differential Revision: D24306098

fbshipit-source-id: be732d22c

jvillard/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer,jvillard/infer,jvillard/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,facebook/infer,jvillard/infer,jvillard/infer