sctp: fix usage of stream scheduler functions

sctp_ss_scheduled() should only be called for streams that are
scheduled. So call sctp_ss_remove_from_stream() before it.
This bug was uncovered by the earlier cleanup.

Reported by: syzbot+bbf739922346659df4b2@syzkaller.appspotmail.com
Reported by: syzbot+0a0857458f4a7b0507c8@syzkaller.appspotmail.com
Reported by: syzbot+a0b62c6107b34a04e54d@syzkaller.appspotmail.com
Reported by: syzbot+0aa0d676429ebcd53299@syzkaller.appspotmail.com
Reported by: syzbot+104cc0c1d3ccf2921c1d@syzkaller.appspotmail.com
MFC after: 1 week