[chore] Remove project board from readme (#32949)

While I personally like the idea of a project board, [it was
closed](https://github.com/orgs/open-telemetry/projects/3) on March 3rd
of this year and not really in use for months (years?) before such. I
believe it's confusing to keep it in the readme, so unless and until we
start managing it again, the reference should be removed.


![image](https://github.com/open-telemetry/opentelemetry-collector-contrib/assets/1878066/e49efd6a-1361-4bfe-9542-61f46913c5b7)

README.md

@@ -137,8 +137,3 @@ The facilitator is not required to perform a thorough review, but they are encou
 enforce Collector best practices and consistency across the codebase and component
 behavior. The facilitators will typically rely on codeowner's detailed review of the code
 when making the final approval decision. 
-
-We recommend maintainers and approvers to keep an eye on the
-[project board](https://github.com/orgs/open-telemetry/projects/3). All newly created
-PRs are automatically added to this board. (If you don't see the PR on the board you
-may need to add it manually by setting the Project field in the PR view).