Simplify Codebase with Native vLLM Weight Update Support

[optimass]

Now that vLLM supports in-flight weight updates, we can eliminate the custom code that previously handled this.
This will lighten the codebase, reduce potential bugs, and make upgrading vLLM versions much simpler.

[ehsk]

Good point, this is our plan to gradually upgrade vLLM to the latest version. We tried this once before but didn't quite work out. Hopefully, we can make it work soon.