Simplify the cache Context.SelectApplications function

In production code, we only use cache Context.SelectApplications in KubernetesShim.schedule with a nil filter.

In testing code, MockScheduler.waitAndAssertApplicationState and MockScheduler.waitAndAssertTaskState use it to filter apps by application ID, but this can be replaced by cache Context.GetApplication.

So we can use a new function GetAllApplications, and remove SelectApplications in the code base.