!swap should remove flags before adding them

[meldra]

If a client times out and their replacement's nick change happens before !swap, the voicing of the new connection gets undone by the bot immediately.

15:19:24 <-- miney (~mineteste@user/minetester) has quit (Ping timeout: 252 seconds)
15:19:24 <@lykos> miney has gone missing.
[...]
15:19:27 -- miney_ is now known as miney
[...]
15:19:49 -- Mode #werewolf [+v-v miney miney] by lykos
15:19:50 <@lykos> miney has swapped places with miney.