sd-device-enumerator: drop /sys/subsystem support

sd-device-enumerator: drop /sys/subsystem support

This addresses the comment by Lennart
https://github.com/systemd/systemd/pull/22662#discussion_r829799863:
> /sys/subsystem is preparation for a future that never came.
> And given that the main proponent of this left Linux kernel
> development (Kay), I doubt this will ever come. So maybe we
> should start dropping references to /sys/subsystem/ given it's
> unlikely to materialize anytime soon.