drm/amdkfd: Remove set but not used variable 'preempt_all_queues'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdkfd/kfd_device_queue_manager.c: In function 'destroy_queue_cpsch':
drivers/gpu/drm/amd/amdkfd/kfd_device_queue_manager.c:1366:7: warning:
 variable 'preempt_all_queues' set but not used [-Wunused-but-set-variable]

It never used since introduct in
commit 992839ad

 ("drm/amdkfd: Add static user-mode queues support")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>