Remove unneeded if and main_thread param from flecs_run_pipeline_ops

[ZeroErrors]

I was looking at changes from #1003 again and realised there was some redundant code that could be cleaned up.

Changelog

• Removed unneeded if (main_thread || op->multi_threaded) from flecs_run_pipeline_ops
  • This isn't needed since flecs_run_pipeline only signals workers when op->multi_threaded is set.
  • I turned this into an assert instead just to make sure it doesn't happen.
• Removed bool main_thread parameter from flecs_run_pipeline_ops since it is not needed.

[SanderMertens]

Hm, how does this work in multithreaded worlds with systems that aren't multithreaded? 🤔 Those should only be ran on the main thread.

[SanderMertens]

Ah, I think I see how this works, the worker threads aren't signaled when the active part of the schedule isn't multithreaded- cool! LGTM 🚀