Don't advertise the use of "ci skip" in the CONTRIBUTING guide

What does this MR do?

It removes the "advertising" of [ci skip] in the CONTRIBUTING guide.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Sometimes, people skip specs when they shouldn't (like when views are modified, specs can still break), and since we have autoscaling, there's no point being cheap about our runners' usage anymore.

What are the relevant issue numbers?

No issue... 👀

/cc @axil