Removed unecessary invocations of org-agenda-show.

Removed unecessary invocations of org-agenda-show.

lisp/org-agenda.el (org-agenda-set-tags): Remove org-agenda-show to prevent
disrupting windows and changing point in original buffer.
(org-agenda-set-property): Same
(org-agenda-set-effort): Same
(org-agenda-toggle-archive-tag): Same

When setting a tag in the agenda, org-mode displays the corresponding
entry in the original org buffer by calling org-agenda-show. This has
the unwelcome side-effect of disrupting the current window arrangement
and changing the position of the point in the original buffer. This
behavior is inconsistent with the that of org-agenda-todo, which makes
all its changes "silently."

Here is the offending line (6799) in org-agenda-set-tags:

--8<---------------cut here---------------start------------->8---
(org-agenda-show)   ;;; FIXME This is a stupid hack and should not be needed
--8<---------------cut here---------------end--------------->8---

The same line occurs in org-agenda-set-property, org-agenda-set-effort,
and org-agenda-toggle-archive tag.