media: cx18: Remove unused cx18_reset_ir_gpio

cx18_reset_ir_gpio() has been unused in tree since 2009
commit eefe1010 ("V4L/DVB (10759): cx18: Convert GPIO connected
functions to act as v4l2_subdevices")

It has a comment saying it's exported for use by 'lirc_pvr150' but I don't
see any sign of it in the lirc git, and I see it removed support
for lirc_i2c.c 'Flavors of the Hauppage PVR-150...' in 2014.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>