Don't sleep when shutting down tiered storage server thread

WT-8836 added a called to __wt_sleep() when shutting down the tiered storage server thread as part of WT_CONNECTION::close(). This caused a performance regression in tests of initial sync because initial sync closes and reopens WT several times.  

This ticket will remove that sleep. This won't be a problem for now since we don't enable tiered storage outside of a few tests.

I have created WT-9013 to decide whether/how to address the coordination that we were trying to provide with that sleep.