Remove Twitter module

- Twitter does not support OAuth 2[1]

  The motivating issue was created without realizing this (it is a bit
  surprising), and the module that was committed doesn't appear to work.

- For OAuth 1, support[2] already exists

Fixes #25

[1]: https://twittercommunity.com/t/oauth-2-0-support/253
[2]: http://hackage.haskell.org/package/yesod-auth-oauth-1.4.0.2/docs/Yesod-Auth-OAuth.html

Yesod/Auth/OAuth2/Twitter.hs

@@ -1,53 +0,0 @@
-{-# LANGUAGE CPP #-}
-{-# LANGUAGE OverloadedStrings #-}
-module Yesod.Auth.OAuth2.Twitter
-    ( oauth2Twitter
-    , module Yesod.Auth.OAuth2
-    ) where
-
-#if __GLASGOW_HASKELL__ < 710
-import Control.Applicative ((<$>), (<*>))
-#endif
-
-import Control.Monad (mzero)
-import Data.Aeson
-import Data.Text (Text)
-import Data.Text.Encoding (encodeUtf8)
-import Yesod.Auth
-import Yesod.Auth.OAuth2
-
-data TwitterUser = TwitterUser
-    { twitterUserId :: Text
-    , twitterUserName :: Text
-    , twitterScreenName :: Text
-    }
-
-instance FromJSON TwitterUser where
-    parseJSON (Object o) = TwitterUser
-        <$> o .: "id_str"
-        <*> o .: "name"
-        <*> o .: "screen_name"
-
-    parseJSON _ = mzero
-
-oauth2Twitter :: YesodAuth m
-              => Text -- ^ Client ID
-              -> Text -- ^ Client Secret
-              -> AuthPlugin m
-oauth2Twitter clientId clientSecret = authOAuth2 "twitter"
-    OAuth2
-        { oauthClientId = encodeUtf8 clientId
-        , oauthClientSecret = encodeUtf8 clientSecret
-        , oauthOAuthorizeEndpoint = "https://api.twitter.com/oauth/authorize"
-        , oauthAccessTokenEndpoint = "https://api.twitter.com/oauth/access_token"
-        , oauthCallback = Nothing
-        }
-    $ fromProfileURL "twitter" "https://api.twitter.com/1.1/account/verify_credentials.json"
-    $ \user -> Creds
-        { credsPlugin = "twitter"
-        , credsIdent = twitterUserId user
-        , credsExtra =
-            [ ("name", twitterUserName user)
-            , ("screen_name", twitterScreenName user)
-            ]
-        }

yesod-auth-oauth2.cabal

@@ -43,7 +43,6 @@ library
                      Yesod.Auth.OAuth2.Github
                      Yesod.Auth.OAuth2.Google
                      Yesod.Auth.OAuth2.Spotify
-                     Yesod.Auth.OAuth2.Twitter
                      Yesod.Auth.OAuth2.Upcase
                      Yesod.Auth.OAuth2.EveOnline
                      Yesod.Auth.OAuth2.Nylas