chore: remove `async_executor` param in ToolInvoker

[Amnah199]

With the update introduced in PR #9530, the async_executor parameter has been deprecated in ToolInvoker. As a result:

• The async_executor argument should now be removed from the __init__ method.

• The internal flag self._owns_executor is no longer necessary and should also be removed as part of this cleanup.

This aligns with the new approach of using the max_workers parameter to manage parallel execution via a default ThreadPoolExecutor.

Checklist

Tasks

• The changes are merged in the main branch (Code + Docstrings)
• Release notes have documented the breaking change
• A new version of haystack-ai has been released on PyPI
• Docs at https://docs.haystack.deepset.ai/ were updated
• Integrations on haystack-core-integrations were updated (if needed) - This step might require a Breaking change proposal on the repo
• Notebooks on https://github.com/deepset-ai/haystack-cookbook were updated (if needed)
• Tutorials on https://github.com/deepset-ai/haystack-tutorials were updated (if needed)
• Articles on https://github.com/deepset-ai/haystack-home/tree/main/content were updated (if needed)
• Integration tile on https://github.com/deepset-ai/haystack-integrations was updated (if needed)