Revert "Port 'external startup data' flag from gyp to gn."

This cause compile failures on the GN buildbot about 'gen/v8/libraries.bin'
missing and no known rule to make it.

TBR=vogelheim@chromium.org

Review URL: https://codereview.chromium.org/327703004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21739 ce2b1a6d-e550-0410-aec6-3dcde31c8c00