Remove support for internal tiered manager thread

The original POC code considered the flush_tier code to be similar to checkpoints and created an API that is similar. However, it seems like the expectation long term is that the user (MongoDB) wants to control when a flush_tier happens. The POC code contains a lot of code for tiered_manager that seems unlikely to be used.

This ticket should consider if that API is necessary and if it is not, the code relating to tiered_manager should be removed. This would simplify the subsystem and remove code.

Having less variations to test and different ways to use tiered storage is a benefit.