RFR (S) 8080298: Clean up os::...::supports_variable_stack_size()

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8080298

webrev: http://cr.openjdk.java.net/~dholmes/8080298/webrev/

To support the old LinuxThreads implementation we had to distinguish 
between threading libraries with fixed-stack-size threads 
(LinuxThreads), and variable-stack-sized-threads (NPTL). As LinuxThreads 
support was removed years ago and all the code related to it has now 
been removed, we have a situation where supports_variable_stack_size() 
is always true and so the function and its use can be removed.

While this notion was only ever relevant to Linux it also got copied 
across to the BSD and AIX ports.

Thanks,
David