drm/sched/tests: Remove relict of done_list

A rework of the scheduler unit tests removed the done_list. That list is
still mentioned in the mock test header.

Remove that relict.

Fixes: 4576de9b7977 ("drm/sched/tests: Implement cancel_job() callback")
Reviewed-by: Tvrtko Ursulin <[e-mail]>
Signed-off-by: Philipp Stanner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/scheduler/tests/sched_tests.h

···
  *
  * @base: DRM scheduler base class
  * @test: Backpointer to owning the kunit test case
- * @lock: Lock to protect the simulated @hw_timeline, @job_list and @done_list
+ * @lock: Lock to protect the simulated @hw_timeline and @job_list
  * @job_list: List of jobs submitted to the mock GPU
- * @done_list: List of jobs completed by the mock GPU
  * @hw_timeline: Simulated hardware timeline has a @context, @next_seqno and
  *		 @cur_seqno for implementing a struct dma_fence signaling the
  *		 simulated job completion.