usb: gadget: Kconfig: Remove unused option for RMNET_SMD_CTRL

Starting kernel 3.0, all USB function and transport drivers are
compiled in the kernel and user can enable the required functions/
transports using sysfs attributes. Hence, remove the unused
Kconfig option which is no longer used anywhere.

Change-Id: I9ac9e14c2dd95827f7e0fd48393a24552cd980d1
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>