V4L/DVB (6504): pvrusb2: Remove dead sysfs code

The pvrusb2 driver's sysfs implementation had long since implemented a
dummy hotplug function because at the time the kernel would oops
without at least the empty function being present.  Today - after
numerous class interface changes in the kernel - this pvrusb2 change
had been dutifully carried forward but an inspection of the kernel
sources shows that it is no longer needed.  So remove the dummy
function and its reference.  This also solves a recurring backwards
compatibility issue in the pvrusb2 driver as the class interface has
been getting thrashed in the kernel.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>