remove unused openbsd std::isfinite and std::signbit implementations
remove unused openbsd std::isfinite and std::signbit implementations

OpenBSD 4.4 (release 2008) and later include std::isfinite and std::signbit.  

https://bugs.webkit.org/show_bug.cgi?id=19975

Review URL: https://codereview.chromium.org/804943002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187166 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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