Clear master cx export maps
Clear master cx export maps

Summary:
We merge these from the master context, but forgot to clear them like we do
other parts like the tvar graph and property maps.

Reviewed By: panagosg7

Differential Revision: D18129296

fbshipit-source-id: 1020c68d990265698600f19fed22b72955f0246f

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