refactor(datastore): remove unnecessary methods from repository store
This is a preparatory cleanup/refactor MR to facilitate the upcoming changes for #461 (closed).
While working on a draft solution I realized that the current "single repository cache" solution had a Clear method whose only purpose was to satisfy an existing test for the Delete method on the repository store. I then realized this Delete method is not used anywhere, so it's not necessary. This means we can get rid of both.
Additionally, The FindByID method of the repository store is also unused except for testing. We lookup repositories by path everywhere else, so I've converted the tests to use FindByPath instead and removed the former method as well.