[finsh] remove finsh_syscall_lookup() function declaration

components/finsh/finsh.h

@@ -240,9 +240,6 @@ extern struct finsh_syscall *_syscall_table_begin, *_syscall_table_end;
     #define FINSH_NEXT_SYSCALL(index)  index++
 #endif
 
-/* find out system call, which should be implemented in user program */
-struct finsh_syscall *finsh_syscall_lookup(const char *name);
-
 #if !defined(RT_USING_POSIX_STDIO) && defined(RT_USING_DEVICE)
 void finsh_set_device(const char *device_name);
 #endif