drm/sched/tests: Remove redundant header files

The header file <linux/atomic.h> is already included on line 8. Remove the
redundant include.

Fixes: 5a99350794fec ("drm/sched: Add scheduler unit testing infrastructure and some basic tests")
Signed-off-by: Liao Yuanhong <[e-mail]>
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

···
 #include <linux/hrtimer.h>
 #include <linux/ktime.h>
 #include <linux/list.h>
-#include <linux/atomic.h>
 #include <linux/mutex.h>
 #include <linux/types.h>