Remove gpu/demos/demos.gyp from build/all.gyp.
Remove gpu/demos/demos.gyp from build/all.gyp.

These are supposed to be built with the nacl toolchain. Building them
with the normal toolchain as part of `ninja All` isn't useful.

BUG=none
TEST=none


Review URL: http://codereview.chromium.org/9430015

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@122893 0039d316-1c4b-4281-b951-d872f2087c98

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