playlist-remove has a required index argument, as indicated by the
documentation. However, in 332907e1d7
MP_CMD_OPT_ARG was mistakenly added, making it optional.
The effect of not specifying the argument is defaulting the index to 0
and always removing the first element in the playlist, which is not
expected.
Fix this by making the argument required.
Fixes: 332907e1d7