HID: wacom: remove unused hid_data::pressure

The pressure member in struct hid_data is unused. It was
added in commit 5ae6e89f (HID: wacom: implement the finger part of
the HID generic handling), but never used.

As this is not a struct to communicate with the HW, remove that member.

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

.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Ping Cheng <ping.cheng@wacom.com>
Cc: Jason Gerecke <jason.gerecke@wacom.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>