Fix likely cause of `smarts.del` call in `ros_driver`. (#2106) #9183

Job	Run time
base-tests (./cli)	2m 43s
base-tests (./envision)	3m 13s
base-tests (./smarts/core --nb-exec-timeout 65536 --ignore=./smarts/core/tests/test_notebook.py)	4m 26s
benchmark (e10_drive)	3m 16s
base-tests (./smarts/env --ignore=./smarts/env/tests/test_rllib_hiway_env.py)	8m 54s
benchmark (e11_platoon)	3m 47s
base-tests (./smarts/env/tests/test_rllib_hiway_env.py)	4m 36s
base-tests (./smarts/sstudio)	2m 39s
examples-rl (e10_drive)	5m 18s
examples-rl (e11_platoon)	4m 12s
base-tests (./smarts/ray)	3m 11s
base-tests (./examples/tests/test_examples.py)	5m 44s
	51m 59s

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