um: Remove obsoleted declaration for execute_syscall_skas

The execute_syscall_skas() have been removed since
commit e32dacb9 ("[PATCH] uml: system call path cleanup"),
and now it is useless, so remove it.
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Richard Weinberger <richard@nod.at>

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

@@ -13,7 +13,6 @@ 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 long execute_syscall_skas(void *r);
 extern unsigned long current_stub_stack(void);
 extern struct mm_id *current_mm_id(void);
 extern void current_mm_sync(void);