Remove unnecessary Shadow DOM tests.

input-with-validation-without-shadow.html makes no sense now because validation
message UI implementation doesn't use UA shadow DOM any more.

shadowdom-for-unsupported-elements.html is unnecesary because
shadow-disable.html covers it.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/23977002

git-svn-id: svn://svn.chromium.org/blink/trunk@157271 bbb929c8-8fbe-4397-9dbb-9b2b20218538