Remove references to already deleted graphics/cfg files from gyp
Remove references to already deleted graphics/cfg files from gyp

ImageSourceCG.cpp and PDFDocumentImage.cpp were deleted so we don't
need to exclude them in the core.gyp anymore.

Review URL: https://codereview.chromium.org/13898017

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@148919 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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