De-hardcode link concurrency for Windows.

it's not needed anymore as we're using the GYP_USE_SEPARATE_MSPDBSRV flag on the Windows builders.

BUG=333000, 335607
R=scottmg@chromium.org, robertshield@chromium.org

Review URL: https://codereview.chromium.org/143473002

git-svn-id: http://gyp.googlecode.com/svn/trunk@1842 78cadc50-ecff-11dd-a971-7dbc132099af