usb: gadget: printer: move function-related unbind code to function's unbind

In order to factor out a reusable f_printer.c, the code related to the
function should be placed in functions related to the function.

printer_cfg_unbind() becomes empty, so it is removed.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>