Remove use_system_v8 logic from the mainline gyp file

Re-upload of https://codereview.chromium.org/14872005/

BUG=chromium:226860
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/15001031

Patch from Paweł Hajdan Jr. <phajdan.jr@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14682 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Fix build failure on Linux since r14681.

R=danno@chromium.org, svenpanne@chromium.org

Review URL: https://codereview.chromium.org/14969025

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14683 ce2b1a6d-e550-0410-aec6-3dcde31c8c00