* lisp/org.el (org-effort-duration): Remove unused variable.
* lisp/org-compat.el (org-effort-durations): Make
`org-effort-duration' an alias for `org-duration-units'.
* lisp/org-clock.el (org-clock-modify-effort-estimate): Fix docstring.
* doc/org-manual.org (Footnotes): Clarify footnote.
The variable is inactive since introduction of "org-duration" library.