org.el: Remove `org-link-display-descriptive' and use `org-descriptive-links'.

org.el: Remove `org-link-display-descriptive' and use `org-descriptive-links'.

* org.el (org-link-display-descriptive): Remove this option
and rely on the existing `org-descriptive-links' instead.
(org-toggle-link-display): Use `org-descriptive-links'.

Without this change, both options are redundant.  This also
fixes a bug about using `org-descriptive-links' in the `org-mode'
function, that initialize the display using `org-descriptive-links'
instead of `org-link-display-descriptive'.

Thanks to Thomas S. Dye for spotting this.