drm/xe/bo: Remove unusued variable

bo is not used since all the checks are against tbo. Fix warning:

	../drivers/gpu/drm/xe/xe_bo.c: In function ‘xe_evict_flags’:
	../drivers/gpu/drm/xe/xe_bo.c:250:23: error: variable ‘bo’ set but not used [-Werror=unused-but-set-variable]
	  250 |         struct xe_bo *bo;

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>