Obsolete UseSemaphoreGCThreadsSynchronization

The diagnostic option UseSemaphoreGCThreadsSynchronization was introduced by JDK-8087324 as a way to "back out" the new semaphore-based implementation of starting and stopping GC worker threads in a WorkGang, in case problems with that new implementation were encountered.

Given that the "new" implementation from JDK-8087324 has been the default and used by pretty much everyone since 8/2015, it's time to remove the option and delete the associated code.