users: remove option to disable autocomplete.

This option was originally added in issue #1747. But only ~350 users
ever disabled autocomplete, only ~120 of these were seen in the last
year, and only 9 new users who signed up in the last year disabled it.

Users wishing to disable autocomplete can use this CSS:

    .ui-autocomplete { display: none !important: }

or this Javascript:

    $("[data-autocomplete]").autocomplete("disable");

app/javascript/src/javascripts/posts.js.erb

@@ -117,9 +117,7 @@ Post.open_edit_dialog = function() {
       of: window
     },
     drag: function(e, ui) {
-      if (CurrentUser.data("enable-auto-complete")) {
-        $tag_string.data("uiAutocomplete").close();
-      }
+      $tag_string.data("uiAutocomplete").close();
     },
     close: Post.close_edit_dialog
   });