struct lg3ff_device is present in the driver since its addition by
commit 74f292ca (HID: add driver for the Logitech Flight System
G940).

But it was never used, remove it.

Found by https://github.com/jirislaby/clang-struct.
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Gary Stein <LordCnidarian@gmail.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>