Remove dead code accessing unused simulation_id in measurement callback (#194)

Addresses review feedback on PR #193 regarding unused code in the
`__onmeasurement` callback method.

## Changes

- Removed `message["simulation_id"]` access in
`Simulation.__onmeasurement()` that extracted but never used the value
- The simulation ID is already stored as `self.simulation_id` when the
simulation starts, making this extraction redundant

```python
def __onmeasurement(self, headers, message):
    """Call the measurement callbacks"""
-   message["simulation_id"]  # Dead code
    timestamp = message["message"]["timestamp"]
    measurements = message["message"]["measurements"]
    for p in self.__filterable_measurement_callback_set:
        p[0](self, timestamp, measurements)
```

<!-- START COPILOT CODING AGENT TIPS -->
---

✨ Let Copilot coding agent [set things up for
you](https://github.com/GRIDAPPSD/gridappsd-python/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
— coding agent works faster and does higher quality work when set up for
your repo.

Author: craig8

gridappsd-python-lib/gridappsd/simulation.py

@@ -336,7 +336,6 @@ def __onmeasurement(self, headers, message):
         :param message:
         :return:
         """
-        message["simulation_id"]
         timestamp = message["message"]["timestamp"]
         measurements = message["message"]["measurements"]
         for p in self.__filterable_measurement_callback_set: