doc: remove feature generating html4 and xhtml.

xhtml is dead and we should generate html5, but browsers will properly
display html4.

Author: rouilj

doc/features.txt

@@ -84,7 +84,6 @@ from Ka-Ping Yee in the :index:`Software Carpentry` "Track" design competition.
    list by default
  - flexible access control built around Permissions and Roles with assigned
    Permissions
- - generates valid HTML4 or XHTML
  - detects concurrent user changes
  - saving and editing of user-defined queries which may optionally be
    shared with other users