Remove obsolete OS handling in GN.

tools/gn/settings.cc

Index: tools/gn/settings.cc
diff --git a/tools/gn/settings.cc b/tools/gn/settings.cc
index a5f90d6f21ec95b4eeceec825eeb7fb87227b084..034e601a3476b742b98be657d69e72917e2d3cec 100644
--- a/tools/gn/settings.cc
+++ b/tools/gn/settings.cc
@@ -29,20 +29,6 @@ Settings::Settings(const BuildSettings* build_settings,
   // one-off data without doing generation.
   if (!toolchain_output_dir_.is_null())
     toolchain_gen_dir_ = SourceDir(toolchain_output_dir_.value() + "gen/");
-
-#if defined(OS_WIN)
-  target_os_ = WIN;
-#elif defined(OS_MACOSX)
-  target_os_ = MAC;
-#elif defined(OS_LINUX)
-  target_os_ = LINUX;
-#elif defined(OS_ANDROID)
-  // Currently we don't have an "Android" target OS, it looks just like Linux
-  // from our perspective.
-  target_os_ = LINUX;
-#else
-  #error implement me
-#endif
 }
 
 Settings::~Settings() {