[CSS Grid Layout] Remove legacy WebKitCSSGridLayoutEnabled 
flag from the Layout Tests.

Most of the layout tests come form WebKit codebase before
the fork, so they still have many legacy and unneeded bits.

This patch removes the WebKitCSSGridLayoutEnabled flag 
setting by the testRunner.

BUG=79180

Review URL: https://codereview.chromium.org/180523004

git-svn-id: svn://svn.chromium.org/blink/trunk@168832 bbb929c8-8fbe-4397-9dbb-9b2b20218538