Remove git attributes seed helpers

What does this MR do and why?

#336749 (closed)

Soon we will begin to use the praefect database in tests and when this happens we can no longer rely on being able to directly access repositories without praefect knowing.

There are many more of these to do. I'm just picking off similar parts to package up as MRs to keep the individual review load down.

We will eventually replace all of the seed helpers with the create(:project, :repository) factory.

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