HID: hid-prodikeys: remove unused struct pcmidi_snd members

struct pcmidi_snd contains two unused members: out_substream and
out_active. They were added along the driver in commit 3a370ca1
(HID: Prodikeys PC-MIDI HID Driver), but not used.

Drop them.

Found by https://github.com/jirislaby/clang-struct

.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>