Remove conflicting ActiveRecord association

What does this MR do and why?

Related to #351452 (closed)

While investigating an unrelated behavior I noticed that EE::Project defines a has_many :sourced_pipelines association that has a wrong type. Instead of pointing to Ci::Sources::Pipeline it uses Ci::Sources::Project.

The problem here is not just the naming but the fact that in Core we do have Project#sourced_pipelines and in EE we are overriding this to return a different type.

In this MR we are dropping the EE override rather than renaming it, since it's only used in specs.

I'm wondering whether the actual association to Ci::Sources::Project is of any value today but in the scope of this MR we are only fixing this bug.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.