media: ca.h: get rid of CA_SET_PID

This ioctl seems to be some attempt to support a feature
at the bt8xx dst_ca driver. Yet, as said there, it
"needs more work". Right now, the code there is just
a boilerplate.

At the end of the day, no driver uses this ioctl, nor it is
documented anywhere (except for "needs more work").

So, get rid of it.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>