Removes (annoying) alert on column copy value

All other copy to clipboard action are `alert` free like copying a table name to the clipboard. I think it should be the same for copying a column value.

If you think some feedback is necessary, maybe we should see a small label showing up than closing shortly after. As a comparison, it appears GitHub, in the commit list view, does not shot any feedback anymore (is it a bug maybe? because remote url copy still shows a feedback, anyway :)).

P.S. My editor automatically removed some trailing white spaces, if you prefer, I can revert those.

static/js/app.js

@@ -950,7 +950,6 @@ function bindTableHeaderMenu() {
       switch(menuItem.data("action")) {
         case "copy_value":
           copyToClipboard($(context).text());
-          alert("Requested cell value has been copied to the clipboard!");
           break;
         case "filter_by_value":
           var colIdx   = $(context).data("col");