[media] get_dvb_firmware: it913x: Remove it9137 firmware files

Remove it9137 firmware files it9137.txt and it9137 get_dvb_firmware.

dvb-usb-it9137-01.fw firmware is no longer in use.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>