Drop legacy support for `<input />` tag

The request does not appear to be sent at all.

https://jsfiddle.net/XMAq2/99/

Originally reported on Stack Overflow: [select2 4.0 ajax problems since update from 3.5](http://stackoverflow.com/q/29905444/359284) ([chat](http://chat.stackoverflow.com/rooms/76405/discussion-between-kevin-brown-and-user756659))
