drivers: xds110: Remove unnecessary and deprecated libusb function.

drivers: xds110: Remove unnecessary and deprecated libusb function.

libusb-1.0.22 has deprecated the libusb_set_debug function. This
function was present in the XDS110 driver as an artifact left over
from early debugging. It isn't required because logging is disabled
by default. Removing it to simplify the code and no longer need to
have libusb version specific #if's in the code.

Change-Id: I9c3f16c039da3f3fad9c4a1169978b0d85a1b45c
Signed-off-by: Edward Fewell <efewell@ti.com>
Reviewed-on: http://openocd.zylin.com/4553
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins