Remove MINIMAL pragma for authHttpManager

We now have a default implementation for it. See this for more
information:
https://github.com/yesodweb/yesod/issues/1489#issuecomment-370200663

Helps in preventing warnings like this:

```
serverside.hs:40:10: warning: [-Wmissing-methods]
    • No explicit implementation for
        ‘authHttpManager’
    • In the instance declaration for ‘YesodAuth App’
   |
40 | instance YesodAuth App where
   |          ^^^^^^^^^^^^^
```

yesod-auth/Yesod/Auth.hs

@@ -243,7 +243,7 @@ class (Yesod master, PathPiece (AuthId master), RenderMessage master FormMessage
       man <- authHttpManager
       withRunInIO $ \run -> withResponse req man $ run . inner
 
-    {-# MINIMAL loginDest, logoutDest, (authenticate | getAuthId), authPlugins, authHttpManager #-}
+    {-# MINIMAL loginDest, logoutDest, (authenticate | getAuthId), authPlugins #-}
 
 {-# DEPRECATED getAuthId "Define 'authenticate' instead; 'getAuthId' will be removed in the next major version" #-}