[Serve] Remove extraneous serve.start() in run_graph() task

[shrekris-anyscale]

Why are these changes needed?

The run_graph() task has been manually starting Serve with serve.start() to forcibly select the "serve" namespace. However, #25575 made all Serve actors start in the "serve" namespace, meaning this call is extraneous. serve.run() will call serve.start() anyways. This change removes serve.start() from the run_graph() task.

Related issue number

N/A

Checks

• 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/
• Testing Strategy
  • This change relies on existing tests.