usb: gadget: u_bam: Remove support for BAM-DMUX

BAM-DMUX support has been dropped from this kernel release.
Remove all references to it from USB RmNet/u_bam drivers and
also get rid of redundant variables.

However keep the software RX code path as it is still needed
for the case of the SYS2BAM uplink workaround.

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