Clean up default-platform gyp defines

The files that are excluded do not exist at that location, so the rule
is not effective.

We'll first want to move the platform stuff into a separate library, so
I'm just removing those lines for now

BUG=v8:3015
R=dcarney@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/237053002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20727 ce2b1a6d-e550-0410-aec6-3dcde31c8c00