CLEANUP: Remove prototype for non-existent thread_get_default_count()

This is the only location of `thread_get_default_count` within the codebase.
diff --git a/include/haproxy/thread.h b/include/haproxy/thread.h
index 5c0dc49..a06b93d 100644
--- a/include/haproxy/thread.h
+++ b/include/haproxy/thread.h
@@ -46,7 +46,6 @@
 
 /* Generic exports */
 int parse_nbthread(const char *arg, char **err);
-int thread_get_default_count();
 extern int thread_cpus_enabled_at_boot;