Mark package-x.el as obsolete

* lisp/obsolete/package-x.el: Add "Obsolete-since" header.  (Bug#76194)
* doc/lispref/package.texi (Package Archives): Don't document package-x.

doc/lispref/package.texi

@@ -333,12 +333,6 @@ and associated files, are stored.  If you want the archive to be
 reachable via HTTP, this directory must be accessible to a web server;
 @xref{Archive Web Server}.
 
-  A convenient way to set up and update a package archive is via the
-@code{package-x} library.  This is included with Emacs, but not loaded
-by default; type @kbd{M-x load-library @key{RET} package-x @key{RET}} to
-load it, or add @code{(require 'package-x)} to your init file.
-@xref{Lisp Libraries,, Lisp Libraries, emacs, The GNU Emacs Manual}.
-
 @noindent
 After you create an archive, remember that it is not accessible in the
 Package Menu interface unless it is in @code{package-archives}.

etc/NEWS

@@ -1130,6 +1130,9 @@ will not prompt the user for confirmation before installing packages.
 Refreshing the package index will no longer block when invoked
 interactively.
 
++++
+*** package-x.el is now obsolete.
+
 ** Xref
 
 ---

lisp/obsolete/package-x.el

@@ -6,6 +6,7 @@
 ;; Created: 10 Mar 2007
 ;; Keywords: tools
 ;; Package: package
+;; Obsolete-since: 31.1
 
 ;; This file is part of GNU Emacs.