usb: gadget: f_serial: Cleanup dead code related to DUN

Commit e96fc1ee ("usb: gadget: f_serial: Add support for DUN")
introduced several members in gserial structure but they
are not really used since u_serial does not support TIOCMSET/TIOCMGET
tty ioctls. Drop the dead code so that our generic serial implementation
in f_serial compliant to GKI.

Change-Id: I53fcc77c737f9c0ce021713ec024d36a73c143b8
Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>