Remove now-invalid reference to graph extra from docs (#873)

Author: dmontagu

docs/install.md

@@ -47,7 +47,6 @@ pip/uv-add 'pydantic-ai-slim[openai]'
 `pydantic-ai-slim` has the following optional groups:
 
 * `logfire` — installs [`logfire`](logfire.md) [PyPI ↗](https://pypi.org/project/logfire){:target="_blank"}
-* `graph` - installs [`pydantic-graph`](graph.md) [PyPI ↗](https://pypi.org/project/pydantic-graph){:target="_blank"}
 * `openai` — installs `openai` [PyPI ↗](https://pypi.org/project/openai){:target="_blank"}
 * `vertexai` — installs `google-auth` [PyPI ↗](https://pypi.org/project/google-auth){:target="_blank"} and `requests` [PyPI ↗](https://pypi.org/project/requests){:target="_blank"}
 * `anthropic` — installs `anthropic` [PyPI ↗](https://pypi.org/project/anthropic){:target="_blank"}