Remove metadata creation hack from Praefect

Praefect contains a testing hack that created metadata for repositories when Praefect first received a request for them. This allowed Praefect to be used in Gitaly tests that placed repositories directly in the storage rather than creating them through the API. Placing the repositories directly in the storage didn't allow for Praefect to create the required metadata entries on repository creation which in turn caused failures in the tests as Praefect would return a not found. All of the said tests have been changed to create the repositories through the API, so the hack is no longer needed. This commit removes the hack from Praefect and the associated configuration options.

Closes #4019 (closed)