Remove unused imports. #9191

Job	Run time
benchmark (e10_drive)	3m 21s
examples-rl (e10_drive)	5m 14s
benchmark (e11_platoon)	2m 2s
examples-rl (e11_platoon)	3m 5s
base-tests (./cli)	2m 53s
base-tests (./envision)	3m 23s
base-tests (./smarts/core --nb-exec-timeout 65536 --ignore=./smarts/core/tests/test_notebook.py)	4m 30s
base-tests (./smarts/env --ignore=./smarts/env/tests/test_rllib_hiway_env.py)	4m 48s
base-tests (./smarts/env/tests/test_rllib_hiway_env.py)	3m 13s
base-tests (./smarts/sstudio)	2m 45s
base-tests (./smarts/ray)	3m 12s
base-tests (./examples/tests/test_examples.py)	5m 55s
	44m 21s

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