Remove redundant crash tests using Selection.modify("-webkit-visual-word").

We removed experimental flag -webkit-visual-word 2 years ago:
http://src.chromium.org/viewvc/blink/trunk/Source/core/page/DOMSelection.cpp?r1=89440&r2=102252
Selection.modify("-webkit-visual-word") does nothing.
However, there are still tests using the flag.
They confirm that scripts don't crash when they call Selection.modify("-webkit-visual-word").
It doesn't make sense.

BUG=

Review URL: https://codereview.chromium.org/236733005

git-svn-id: svn://svn.chromium.org/blink/trunk@171453 bbb929c8-8fbe-4397-9dbb-9b2b20218538