V4L/DVB: ir-core: remove IR_TYPE_PD

Pulse-distance is not a protocol, it is a line coding (used by some protocols,
like NEC). Looking at the uses of IR_TYPE_PD, the real protocol seems to be
NEC in all cases (drivers/media/video/cx88/cx88-input.c is the only user).

So, remove IR_TYPE_PD while it is still easy to do so.

Signed-off-by: David Härdeman <david@hardeman.nu>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>