feat: remove /modmail_setup command

It doesn't work as intended since if you delete the modmail message, the previous deleted message ID is still stored in the data struct, and so the command thinks it's still valid and does nothing.

The way to setup the modmail message it is to delete the modmail message and restart the bot, because it automatically configures the modmail on startup.

src/commands/modmail.rs

@@ -88,19 +88,6 @@ pub async fn modmail(
 	Ok(())
 }
 
-#[poise::command(
-	prefix_command,
-	slash_command,
-	ephemeral,
-	category = "Modmail",
-	hide_in_help,
-	check = "crate::checks::check_is_moderator"
-)]
-pub async fn modmail_setup(ctx: Context<'_>) -> Result<(), Error> {
-	load_or_create_modmail_message(ctx, ctx.data()).await?;
-	Ok(())
-}
-
 pub async fn load_or_create_modmail_message(
 	http: impl serenity::CacheHttp,
 	data: &Data,