XMLErrors::insertErrorMessageBlock does not need to call detach

parserRemoveChild already does a detach(), there's no reason for
insertErrorMessageBlock to do it.

Review URL: https://chromiumcodereview.appspot.com/23493004

git-svn-id: svn://svn.chromium.org/blink/trunk@156759 bbb929c8-8fbe-4397-9dbb-9b2b20218538