[CSS Grid Layout] Remove unused variable in test

Inside method updateImplicitGridColumn() of test
fast/css-grid-layout/implicit-position-dynamic-change.html we define a
variable gridItem that is not used. This patch removes it.

Review URL: https://codereview.chromium.org/213423009

git-svn-id: svn://svn.chromium.org/blink/trunk@170301 bbb929c8-8fbe-4397-9dbb-9b2b20218538