Remove useless (t nil) sexps at the end of some (cond ...) constructs

Remove useless (t nil) sexps at the end of some (cond ...) constructs

* org.el (org-compute-latex-and-specials-regexp)
(org-paste-subtree, org-sort-entries, org-store-link)
(org-open-at-point, org-file-remote-p, org-add-log-setup)
(org-set-tags-to, org-fast-tag-selection)
(org-diary-sexp-entry): Ditto.

* org-agenda.el (org-agenda-get-blocks, org-cmp-priority)
(org-cmp-effort, org-cmp-todo-state, org-cmp-alpha)
(org-cmp-tag, org-cmp-time): Remove useless (t nil) sexps at
the end of (cond ...) constructs.

* org-mobile.el (org-mobile-create-index-file): Ditto.

* org-lparse.el (org-lparse-format-table-row): Ditto.

* org-list.el (org-sort-list): Ditto.

* org-id.el (org-id-get): Ditto.

* org-html.el (org-export-html-preprocess): Ditto.

* org-exp.el (org-default-export-plist)
(org-table-clean-before-export): Ditto.

(t nil) in (cond (...) (...) (t nil)) has no other meaning that to
remind the developer that the cond sexp returns nil in case no condition
is matched.  For several (cond ...) constructs this is obvious from reading
the code.  For others, the reminder might be useful and we leave it.

See the discussion about this on emacs-devel:
http://thread.gmane.org/gmane.emacs.devel/152664