drm/sched: Remove kthread header
The kthread header doesn't need to be included anymore. It's a relict
from commit a6149f039369 ("drm/sched: Convert drm scheduler to use a
work queue rather than kthread").
Remove the unneeded includes.
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Signed-off-by: Philipp Stanner <phasta@kernel.org>
Link: https://lore.kernel.org/r/20250314101023.111248-3-phasta@kernel.org