JSBigString.h: Remove duplicate include of 'unistd.h' (#23297)
JSBigString.h: Remove duplicate include of 'unistd.h' (#23297)

Summary:
This removes the accidental double include of the header `unistd.h` from `JSBigString.h`. One was added by me as part of #22330, and one by matthargett as part of #21764

[General] [Fixed] - `JSBigString.h`: Removed accidental double include of header `unistd.h`
Pull Request resolved: https://github.com/facebook/react-native/pull/23297

Differential Revision: D13961223

Pulled By: hramos

fbshipit-source-id: c0dba8a475b3c09356d34cb65b989c286793fa67

pandiaraj44/react-native,facebook/react-native,arthuralee/react-native,hoangpham95/react-native,pandiaraj44/react-native,hammerandchisel/react-native,exponent/react-native,exponentjs/react-native,exponentjs/react-native,exponentjs/react-native,javache/react-native,pandiaraj44/react-native,janicduplessis/react-native,exponent/react-native,exponentjs/react-native,myntra/react-native,javache/react-native,hoangpham95/react-native,javache/react-native,pandiaraj44/react-native,javache/react-native,myntra/react-native,exponentjs/react-native,facebook/react-native,janicduplessis/react-native,facebook/react-native,exponentjs/react-native,hammerandchisel/react-native,myntra/react-native,janicduplessis/react-native,myntra/react-native,facebook/react-native,arthuralee/react-native,exponentjs/react-native,hoangpham95/react-native,janicduplessis/react-native,pandiaraj44/react-native,myntra/react-native,janicduplessis/react-native,pandiaraj44/react-native,exponent/react-native,facebook/react-native,exponentjs/react-native,exponent/react-native,hammerandchisel/react-native,facebook/react-native,janicduplessis/react-native,javache/react-native,myntra/react-native,hammerandchisel/react-native,hammerandchisel/react-native,javache/react-native,janicduplessis/react-native,hammerandchisel/react-native,hoangpham95/react-native,javache/react-native,javache/react-native,pandiaraj44/react-native,myntra/react-native,hoangpham95/react-native,exponent/react-native,facebook/react-native,arthuralee/react-native,javache/react-native,myntra/react-native,exponent/react-native,arthuralee/react-native,pandiaraj44/react-native,arthuralee/react-native,myntra/react-native,hoangpham95/react-native,hammerandchisel/react-native,exponent/react-native,janicduplessis/react-native,hammerandchisel/react-native,hoangpham95/react-native,facebook/react-native,facebook/react-native,exponent/react-native,hoangpham95/react-native