xcode_emulation: Do not redirect stderr when calling xcodebuild.

For someone, `xcodebuild -version -sdk macosx10.6 Path` would print

    2013-04-12 17:18:59.803 xcodebuild[39147:f07] Error loading /Users/alexisme/Library/Developer/Xcode/Third-Party Plug-ins/CodePilot2.xcplugin/Contents/MacOS/CodePilot2:  dlopen(/Users/alexisme/Library/Developer/Xcode/Third-Party Plug-ins/CodePilot2.xcplugin/Contents/MacOS/CodePilot2, 265): Symbol not found: _IDEEditorDocumentDidChangeNotification
      Referenced from: /Users/alexisme/Library/Developer/Xcode/Third-Party Plug-ins/CodePilot2.xcplugin/Contents/MacOS/CodePilot2
      Expected in: flat namespace
     in /Users/alexisme/Library/Developer/Xcode/Third-Party Plug-ins/CodePilot2.xcplugin/Contents/MacOS/CodePilot2
    2013-04-12 17:18:59.804 xcodebuild[39147:f07] WARNING: Failed to load plugin at: /Users/alexisme/Library/Developer/Xcode/Third-Party Plug-ins/CodePilot2.xcplugin, skipping.  Could not load bundle.

to stderr. This was captured too and got added to the sdk path, which caused
a hard-to-understand exception later one. There's no reason to redirect
stderr, just let it print to the console instead.
Review URL: https://codereview.chromium.org/14163004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1611 78cadc50-ecff-11dd-a971-7dbc132099af