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: http://v8.googlecode.com/svn/branches/bleeding_edge@14682 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/common.gypi

@@ -29,7 +29,6 @@
 
 {
   'variables': {
-    'use_system_v8%': 0,
     'msvs_use_common_release': 0,
     'gcc_version%': 'unknown',
     'CXX%': '${CXX:-$(which g++)}',  # Used to assemble a shell command.