Remove useless calls to DOMImplementation::hasFeature("XML", String())

These calls will always return true, so this was dead code.

BUG=none

Review URL: https://codereview.chromium.org/254423002

git-svn-id: svn://svn.chromium.org/blink/trunk@172483 bbb929c8-8fbe-4397-9dbb-9b2b20218538