chore: remove vector_db_id from AgentSessionInfo (#1296)

# What does this PR do?

- It is not being used anywhere and doesn't make sense to have 1 single
vector_db_id in an agent session. No top level API change.
- See
https://github.com/meta-llama/llama-stack/pull/1286#discussion_r1972569881

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan

- See
https://github.com/meta-llama/llama-stack/pull/1286#discussion_r1972569881

[//]: # (## Documentation)

llama_stack/providers/inline/agents/meta_reference/persistence.py

@@ -21,8 +21,6 @@ log = logging.getLogger(__name__)
 class AgentSessionInfo(BaseModel):
     session_id: str
     session_name: str
-    # TODO: is this used anywhere?
-    vector_db_id: Optional[str] = None
     started_at: datetime