Remove everything region-specific from LayoutTests.

The CSS regions feature is being removed (in an upcoming CL).

BUG=none

Review URL: https://codereview.chromium.org/159933010

git-svn-id: svn://svn.chromium.org/blink/trunk@167048 bbb929c8-8fbe-4397-9dbb-9b2b20218538