w32pthreads: always use Vista+ API, drop XP support

This removes the XP compatibility code, and switches entirely to SRW
locks, which are available starting at Windows Vista.

This removes CRITICAL_SECTION use, which allows us to add
PTHREAD_MUTEX_INITIALIZER, which will be useful later.

Windows XP is hereby not a supported build target anymore.

Signed-off-by: Diego Biurrun <diego@biurrun.de>

libavfilter/pthread.c

@@ -200,10 +200,6 @@ int ff_graph_thread_init(AVFilterGraph *graph)
 {
     int ret;
 
-#if HAVE_W32THREADS
-    w32thread_init();
-#endif
-
     if (graph->nb_threads == 1) {
         graph->thread_type = 0;
         return 0;