drm/radeon: remove set but not used variable 'rdev'

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

drivers/gpu/drm/radeon/radeon_object.c: In function 'radeon_bo_unref':
drivers/gpu/drm/radeon/radeon_object.c:317:24: warning:
 variable 'rdev' set but not used [-Wunused-but-set-variable]

It not used  any more after commit
  e7e31600 ("drm/radeon: remove taking mclk_lock from radeon_bo_unref")
Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_object.c

@@ -314,11 +314,9 @@ struct radeon_bo *radeon_bo_ref(struct radeon_bo *bo)
 void radeon_bo_unref(struct radeon_bo **bo)
 {
 	struct ttm_buffer_object *tbo;
-	struct radeon_device *rdev;
 
 	if ((*bo) == NULL)
 		return;
-	rdev = (*bo)->rdev;
 	tbo = &((*bo)->tbo);
 	ttm_bo_put(tbo);
 	*bo = NULL;