libwacom has been unsetting ID_INPUT_JOYSTICK for known tablets since 2015
(libwacom 0.12) so this comment is outdated. And the input-id udev builtin
never labels something as tablet *and* joystick. Which means: systemd sets
either tablet or joystick. For tablets that are known to libwacom the joystick
bit gets corrected and we only see the tablet bits.

Tablets unknown to libwacom remain as joysticks and are ignored but that's the
behavior we had anyway.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>