[tune] avoid system exit error

[richardliaw]

Why are these changes needed?

Use del instead of __ray_terminate__.

Related issue number

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• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
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[richardliaw]

@rkooo567 can you help verify this is the right way to terminate actors?

[rkooo567]

This will work if we can ensure this is the last reference of this actor. Why not just using ray.kill? (do you need graceful shutdown?)

[richardliaw]

@rkooo567 yes, graceful termination is strictly necessary!