Revert "usb: gadget: f_mtp: Handle mtp os desc properly"

This reverts commit ac9d7c8b.
The fi->f pointer is getting assigned during function allocation
However, if MTP function is not added to the configuration, it
will still get control packets routed to its setup function
mtp_ctrlreq_configfs() and will dereference a NULL f->config.

Since fi->f is already correctly getting assigned during bind
and cleared during unbind, there is no need to assign it
during allocation.

Change-Id: Ic7c6a29796dd62774d36ad029cc0b854a42a8a4a
Signed-off-by: Jack Pham <jackp@codeaurora.org>