Remove Koa2-OAS3 from IMPLEMENTATIONS.md (#2708)
Remove Koa2-OAS3 from IMPLEMENTATIONS.md (#2708)

Project's url https://github.com/OverSpeedIO/koa2-oas3 shows 404. Can't find the project through google either. Looks like the project closed.
OAI/OpenAPI-Specification,OAI/OpenAPI-Specification,OAI/OpenAPI-Specification