Clean up --experimental_remote_mark_tool_inputs

--experimental_remote_mark_tool_inputs is required for remote persistent workers to function correctly:

• https://www.buildbuddy.io/docs/rbe-platforms/#remote-persistent-worker-properties
• https://docs.engflow.com/re/client/remote-persistent-workers.html#client-side-requirements
• https://github.com/buildfarm/buildfarm/blob/127b6c6ad97cc2fc3933aa95646a219c5473598d/src/main/java/build/buildfarm/worker/Executor.java#L488

This flag was first introduced in Bazel 6.0.0, and doesn't describe any reason users should keep it disabled. As-is, this appears to be unnecessary boilerplate for remote persistent workers. One of the following should be done:

1. Enable the flag by default, and target a version for removal.
2. Document the tradeoff that justifies why this flag is off by default.