usb: gadget: Drop support for USB_GADGET_XPORT_BAM2BAM xport

USB_GADGET_XPORT_BAM2BAM was used for A2 <-> USB bam2bam
connections. As this path is no longer supported, drop the dead
code.

Change-Id: Iab030caee7adb46e2bf7e56f3ecf0ef84e2be953
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>