org.texi: Remove reference to `org-effort-property'

* doc/org.texi (Effort estimates): Remove `org-effort-property' from
  variables index since it is a defconst, not a defcustom anymore.

See also 20dcd061a51a8121ff958fc0edb5b7c52ba4e0e0.

doc/org.texi

@@ -6689,7 +6689,6 @@ with @code{org-clock-in} and two @kbd{C-u C-u} with @code{org-clock-in-last}.
 @cindex effort estimates
 
 @cindex property, Effort
-@vindex org-effort-property
 If you want to plan your work in a very detailed way, or if you need to
 produce offers with quotations of the estimated work effort, you may want to
 assign effort estimates to entries.  If you are also clocking your work, you