[media] img-ir/hw: Drop [un]register_decoder declarations

The img_ir_register_decoder() and img_ir_unregister_decoder() functions
were dropped prior to the img-ir driver being applied to simplify the
protocol decoder setup. However the declarations of these functions in
img-ir-hw.h were still included. Delete them since they're completely
unused.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>