Revert "Session and Headers exportet"

This reverts commit f58a695c84.

yesod-core/Yesod/Core.hs

@@ -28,13 +28,11 @@ module Yesod.Core
     , logError
     , logOther
       -- * Sessions
-    , Session
     , SessionBackend (..)
     , defaultClientSessionBackend
     , clientSessionBackend
     , saveClientSession
     , loadClientSession
-    , Header(..)
     -- * JS loaders
     , loadJsYepnope
     , ScriptLoadPosition (..)
@@ -52,7 +50,6 @@ module Yesod.Core
     ) where
 
 import Yesod.Internal.Core
-import Yesod.Internal
 import Yesod.Content
 import Yesod.Dispatch
 import Yesod.Handler