Don't eagerly store articles in the Agent by default

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 f3b146e943cd733fb716c75048f24b73826e5f30, 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’.