ApiParse: Deprecate prop=headitems

ApiParse: Deprecate prop=headitems

This code has been rotting for a while. It is important missing stylesheets
and critical inline scripts. Consuming these items is quite risky.
One can't safely use it to update an existing document (as some parts are
duplicative or conflicting). One also can't use it to build a new document as
several critical pieces are missing.

* Since ResourceLoader was introduced in 2011, buildCssLinksArray() doesn't have any stylesheets.

For most pages, this will produce an empty array for headitems.

Mark it as deprecated since in favour of 'prop=headhtml' or 'prop=modules'.

Change-Id: I6084e4454e245f59bf3c82d6db830977725f71e5