Cleanup tightly_coupled_allocation

During the review of JDK-8263361 an issue with tightly_coupled_allocation perhaps not working as intended was raised.

In JDK-8263361 tightly_coupled_allocation failed because the slow_path wasn't initialized yet. In that bug the call wasn't needed.

But looking at the implementation - there is a slow path handling but it isn't used from any context.

Verify that tightly_coupled_allocation is working in all contexts and remove slow_region parameter and code if it is broken or unused.