video / output: Drop display output class support

It was only ever used by the ACPI video driver, and that only use case
vanished over 3 years ago (see commit 677bd810, "ACPI video: remove
output switching control".) So this is dead code and I guess we can
remove it now.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>