Remove LayoutTests/fast/multicol/newmulticol/cell-shrinkback.html

The ref had become 100% identical to the test (after creation of the
virtual/regionbasedcolumns/ testsuite and removal of calls
to settings.setRegionBasedColumnsEnabled()).

Note that there's also a LayoutTests/fast/multicol/cell-shrinkback.html
which is in the process of being fixed. See issue 397257.

R=pkasting@chromium.org

Review URL: https://codereview.chromium.org/512783002

git-svn-id: svn://svn.chromium.org/blink/trunk@180998 bbb929c8-8fbe-4397-9dbb-9b2b20218538