Remove unneeded iostream include
Remove unneeded iostream include

Summary:
Including `iostream` in a header causes anything that uses this header to gain
an extra static initializer for an `std::ios_base::Init` object.
If there are N translation units in a library that use the header, that's N
more static initializers that need to run.

Reviewed By: yfeldblum

Differential Revision: D4191798

fbshipit-source-id: 65b47401b9177177999cff65a47ac8dffa0e0546

facebook/folly,rklabs/folly,rklabs/folly,floxard/folly,Orvid/folly,charsyam/folly,facebook/folly,charsyam/folly,floxard/folly,Orvid/folly,floxard/folly,facebook/folly,Orvid/folly,Orvid/folly,floxard/folly,floxard/folly,Orvid/folly,charsyam/folly,rklabs/folly,rklabs/folly,facebook/folly,charsyam/folly,facebook/folly,rklabs/folly