Back out "Fixes Fabric related podspec"

Summary:
Original commit changeset: 4b18a931a2e4

The original PR wherein these changes were introduced was problematic. When you expose headers that use C++ from the Yoga podspec, they get automatically imported in the Yoga umbrella file. This causes compilation errors, most likely because it's possible for non-c++ files to import these headers (i.e: RCTConvert.m).

I didn't dig into this too much, but since Fabric still doesn't fully compile in OSS, I think it's reasonable to revert this PR for now. cc Kevin Gozali.

Changelog:
[iOS][Fixed] - Undo Fabric-related podspec change

Reviewed By: fkgozali

Differential Revision: D18284536

fbshipit-source-id: a90454b945af0235424dc56408400cd35efd4e7a