HID: hid-multitouch: remove unused mt_application::dev_time

dev_time was moved to struct mt_application in commit f146d1c4 (HID:
multitouch: Store per collection multitouch data), but is not used since
then. Remove it.

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>