Remove element_call.participant_limit config and associated code.

[Half-Shot]

This is a check added back when Element Call was less stable and full-mesh. These days we applied the limit unevenly, only in the incoming call toasts and the room timeline messages but not in the room header or other places where you might join a call. On balance, I think our need for this has passed and the code can be removed.

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)