Revert "drm/scheduler: track GPU active time per entity"

This reverts commit df622729

 as it introduces a use-after-free,
which isn't easy to fix without going back to the design drawing board.

Reported-by: Danilo Krummrich <dakr@redhat.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>