media: rc: lirc does not use LIRC_CAN_SEND_SCANCODE feature

Since commit 02d742f4 ("media: lirc: lirc daemon fails to detect raw
IR device"), the feature LIRC_CAN_SEND_SCANCODE is no longer used as it
tripped up lircd. The ability to send scancodes for IR Tx is implied by
LIRC_CAN_SEND_PULSE (i.e. any device that can send can use IR Tx encoders).

So, remove LIRC_CAN_SEND_SCANCODE since it never used. This fixes:

Documentation/output/lirc.h.rst:6: WARNING: undefined label:
lirc-can-send-scancode (if the link has no caption the label must precede
a section header

As this flag was added for kernel 4.16, let's remove it, while not too
late.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>