Drop support for XHTML 1.0

Drop support for XHTML 1.0

* $wgHtml5 = false; is now ignored completely.
* $wgDocType and $wgDTD have been removed.
* $wgXhtmlDefaultNamespace is now ignored.
* XHTML5 will be output if $wgMimeType is set to an XML mime type (according to HTML5's rules).
* For backwards compatibility with extensions $wgHtml5 and $wgXhtmlDefaultNamespace are set
  in Setup.php but depending on them is deprecated.

Change-Id: Iad9634e2ee420b5a3bbffe550421fde4fa1819b0