[core] remove import thread

[scv119]

Why are these changes needed?

import thread is no longer needed, remove it.

Related issue number

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[scv119]

@iycheng do we even need function_manager?

[pcmoritz]

There is also join_import_thread in worker.py and def start_import_thread(): that should be removed as well I think :)

Thanks a lot for doing this!!!

[fishbone]

@scv119 function manager is still needed I think. Doc related to this.

Btw, can you confirm that run_functions_on_all_workers have been deprecated? Removing this function will lead that code path failed.

[scv119]

Btw, can you confirm that run_functions_on_all_workers have been deprecated? Removing this function will lead that code path failed.

yup run_functions_on_all_workers is removed from the codebase.

[fishbone]

LGTM