org-agenda: Remove unnecessary visibility modification

org-agenda: Remove unnecessary visibility modification

* lisp/org.el (org-show-context-detail): Change default visibility span
  for agenda context.

* lisp/org-agenda.el (org-agenda-goto):
(org-agenda-todo):
(org-agenda-add-note):
(org-agenda-priority):
(org-agenda-set-tags):
(org-agenda-set-property):
(org-agenda-set-effort):
(org-agenda-toggle-archive-tag):
(org-agenda-clock-in): Special visibility is taken care of by
`org-show-context'.  Do not hard-code anything else.

* contrib/lisp/org-contacts.el (org-contacts-gnus-article-from-goto):
Special visibility is taken care of by `org-show-context'.  Do not
hard-code anything else.