bans: fix exception when user with expired ban logs in.

`ban.destroy` fails because users have many `bans`, not a single `ban`.
Destroying the expired ban isn't necessary anyway.

app/logical/session_loader.rb

@@ -20,7 +20,7 @@ class SessionLoader
     end
     
     if CurrentUser.user
-      CurrentUser.user.unban! if ban_expired?
+      CurrentUser.user.unban! if CurrentUser.user.ban_expired?
     else
       CurrentUser.user = AnonymousUser.new
     end
@@ -86,10 +86,6 @@ private
     session[:user_id] = CurrentUser.user.id
   end
 
-  def ban_expired?
-    CurrentUser.user.is_banned? && CurrentUser.user.recent_ban && CurrentUser.user.recent_ban.expired?
-  end
-
   def cookie_password_hash_valid?
     cookies[:password_hash] && cookies.signed[:user_name] && User.authenticate_cookie_hash(cookies.signed[:user_name], cookies[:password_hash])
   end

app/models/user.rb

@@ -80,6 +80,7 @@ class User < ActiveRecord::Base
   has_many :post_votes
   has_many :bans, lambda {order("bans.id desc")}
   has_one :recent_ban, lambda {order("bans.id desc")}, :class_name => "Ban"
+
   has_one :api_key
   has_one :dmail_filter
   has_one :super_voter
@@ -109,7 +110,10 @@ class User < ActiveRecord::Base
     def unban!
       self.is_banned = false
       save
-      ban.destroy
+    end
+
+    def ban_expired?
+      is_banned? && recent_ban.try(:expired?)
     end
   end