Removed Inbox from core (unused since 4b2a66ed29)

Added the following FIXME:
How should a Twitter user get their Inbox filled with foreign tweets?

Every imported Twitter user has a profile in the Profile table, so we
could setup a Subscription entry for each of those, meaning they get
collected in the InboxNoticeStream... But this would mean a lot of
unnecessary entries and listings that generally just point to the
locked down Twitter service.

Let's figure out a good relation so we can connect any profile to any
imported foreign notice, so it shows up in the "all" feed.

db/core.php

@@ -80,7 +80,6 @@ $classes = array('Schema_version',
                  'Location_namespace',
                  'Login_token',
                  'User_location_prefs',
-                 'Inbox',
                  'User_im_prefs',
                  'Conversation',
                  'Local_group',