More API fixes; remove implicit relationship between hub and user

in APIs.

src/user.h

@@ -146,11 +146,6 @@ extern struct user* user_create(struct hub_info* hub, int sd);
  */
 extern void user_destroy(struct user* user);
 
-/**
- * Will post a message that will delete the user later.
- */
-extern void user_schedule_destroy(struct user* user);
-
 /**
  * Disconnect a user.
  * This will mark the user connection ready for being terminated.