Remove outdated comment about gcc

[epicfaace]

          // Works because `google-closure-compiler-js` is forked in Yarn lockfile.	
          // We can remove this if GCC merges my PR:	
          // https://github.com/google/closure-compiler/pull/2707	
          // and then the compiled version is released via `google-closure-compiler-js`.

It looks like the PR mentioned in that comment was merged quite some time ago, and React is no longer using a forked version of google-closure-compiler-js.

[sizebot]

No significant bundle size changes to report.

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[gaearon]

Thanks