Don't eagerly store articles in the Agent by default

* lisp/gnus/gnus-agent.el (gnus-agent-store-article): Made
obsolete.

* lisp/gnus/gnus-art.el (gnus-request-article-this-buffer): Don't
call it.

* lisp/gnus/gnus-async.el (gnus-async-article-callback): Ditto
(bug#43356).

This partially reverts f3b146e943, which
in was response to what appears to be an erroneous feature-request -
bug#8502.

Instead this should be done instead:

“If I read an article while plugged, do they get entered into the
Agent?”

     *No*. If you want this behavior, add
     ‘gnus-agent-fetch-selected-article’ to
     ‘gnus-select-article-hook’.

lisp/gnus/gnus-agent.el

@@ -3812,6 +3812,7 @@ has been fetched."
         t))))
 
 (defun gnus-agent-store-article (article group)
+  (declare (obsolete nil "28.1"))
   (let* ((gnus-command-method (gnus-find-method-for-group group))
 	 (file (gnus-agent-article-name (number-to-string article) group))
 	 (file-name-coding-system nnmail-pathname-coding-system)

lisp/gnus/gnus-art.el

@@ -7084,10 +7084,7 @@ If given a prefix, show the hidden text instead."
 					      gnus-summary-buffer)
 		    (when gnus-keep-backlog
 		      (gnus-backlog-enter-article
-		       group article (current-buffer)))
-		    (when (and gnus-agent
-			       (gnus-agent-group-covered-p group))
-		      (gnus-agent-store-article article group)))
+		       group article (current-buffer))))
 		  (setq result 'article))
 		 (methods
 		  (setq gnus-override-method (pop methods)))

lisp/gnus/gnus-async.el

@@ -225,12 +225,6 @@ that was fetched."
       (save-excursion
 	(save-restriction
 	  (narrow-to-region mark (point-max))
-	  ;; Put the articles into the agent, if they aren't already.
-	  (when (and gnus-agent
-		     (gnus-agent-group-covered-p group))
-	    (save-restriction
-	      (narrow-to-region mark (point-max))
-	      (gnus-agent-store-article article group)))
 	  ;; Prefetch images for the groups that want that.
 	  (when (fboundp 'gnus-html-prefetch-images)
 	    (gnus-html-prefetch-images summary))