CLEANUP: guid: remove global tree export

guid_tree is not directly used outside of functions provided by the guid
module. Remove its export from the include file.

Author: a-denoyelle

include/haproxy/guid.h

@@ -3,8 +3,6 @@
 
 #include <haproxy/guid-t.h>
 
-extern struct eb_root guid_tree;
-
 void guid_init(struct guid_node *node);
 int guid_insert(enum obj_type *obj_type, const char *uid, char **errmsg);
 void guid_remove(struct guid_node *guid);