API Remove AjaxUniqueTextField, since its operation is very limited (#1947)

It used to validate via ajax, but that has been removed as part
of the 3.0 refactor of all prototype.js code. In the end,
its a field which is quite trivial to implement in custom code
(or in controller code), unless we re-add the Ajax features.
And I think an ajax-validating uniqueness field is too much
of an edge case to belong into core.
Merge pull request #1974 from chillu/pulls/ajaxunique-remove

API Remove AjaxUniqueTextField, since its operation is very limited (#19...