media: pvrusb2: remove unused struct 'debugifc_mask_item'

'debugifc_mask_item' is unused since
commit 681c7399 ("V4L/DVB (6691): pvrusb2: Rework pipeline state
control").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>