gsdx: Don't close threads twice

CloseThread is called in the GSJobQueue destructor, so don't call it
again in the GSThread destructor.

Fixes #392, which was caused by a use after free.

Also prevents pthread_join() from being called twice for each thread
on non-Windows operating systems, which is undefined behaviour.

plugins/GSdx/GSThread.cpp

@@ -36,7 +36,6 @@ GSThread::GSThread()
 
 GSThread::~GSThread()
 {
-	CloseThread();
 }
 
 #ifdef _WIN32