remove redundant declarations of __default_stacksize, __default_guardsize

these are now declared in pthread_impl.h.

src/thread/pthread_attr_init.c

 #include "pthread_impl.h"
 
-extern size_t __default_stacksize;
-extern size_t __default_guardsize;
-
 int pthread_attr_init(pthread_attr_t *a)
 {
 	*a = (pthread_attr_t){0};

src/thread/pthread_create.c

@@ -162,8 +162,6 @@ static void *dummy_tsd[1] = { 0 };
 weak_alias(dummy_tsd, __pthread_tsd_main);
 
 volatile int __block_new_threads = 0;
-extern size_t __default_stacksize;
-extern size_t __default_guardsize;
 
 static FILE *volatile dummy_file = 0;
 weak_alias(dummy_file, __stdin_used);

src/thread/pthread_setattr_default_np.c

@@ -2,9 +2,6 @@
 #include "pthread_impl.h"
 #include <string.h>
 
-extern size_t __default_stacksize;
-extern size_t __default_guardsize;
-
 int pthread_setattr_default_np(const pthread_attr_t *attrp)
 {
 	/* Reject anything in the attr object other than stack/guard size. */