Remove unused Document::openSearchDescriptionURL().
<https://webkit.org/b/126419>

Reviewed by Antti Koivisto.

* dom/Document.h:
* dom/Document.cpp:

    Scrub leftovers from the defunct Chromium port.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@161259 268f45cc-cd09-0410-ab3c-d52691b4dbfc