um: Remove the declaration of user_thread function

This function has never been defined since its declaration was
introduced by commit 1da177e4 ("Linux-2.6.12-rc2").
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/shared/skas/skas.h

@@ -10,7 +10,6 @@
 
 extern int userspace_pid[];
 
-extern int user_thread(unsigned long stack, int flags);
 extern void new_thread_handler(void);
 extern void handle_syscall(struct uml_pt_regs *regs);
 extern unsigned long current_stub_stack(void);