Clears cached available update status data.
Since this function is run at such a low bootstrap level, the Update Manager module is not loaded. So, we can't just call _update_cache_clear(). However, the database is bootstrapped, so we can do a query ourselves to clear out what we want to clear.
Note that we do not want to just truncate the table, since that would remove items related to currently pending fetch attempts.
modules/ installer/ installer.authorize.inc, line 335
- Callbacks and related functions invoked by authorize.php to update projects.