Remove core WordPress test jobs that don’t include vip-go-mu-plugins from vip-go-mu-plugins CircleCi workflow

Core WordPress tests are already validated as running successfully as part of the WordPress release process. Running them without including vip-go-mu-plugins on every change to vip-go-mu-plugins isn’t necessary.

We will continue to run the WordPress core test jobs in CircleCI that include vip-go-mu-plugins on every change to ensure changes work as expected with WordPress core.

https://github.com/Automattic/vip-go-mu-plugins/pull/1910