list_themes() should not call file_exists() on each theme's info file

Currently, before returning the list of themes, the list_themes() function does a file_exists() check on each theme's .info file. If the file doesn't exist, it removes the theme from the returned list.

Issues:

1. This code is called on every non-cached page request, and can't be great for performance (especially since it calls it for every theme on your site, even disabled ones).
2. Imagine some sporadic error that causes file_exists() to return FALSE (filesystem glitch, whatever - i.e. if the files are temporarily unreadable). Then any code that runs on that page request which calls list_themes() will assume your theme simply does not exist. That can have unintended consequences. For example, if you are visiting the block configuration page on that request (or clearing caches, etc), _block_rehash() will run and will disable all blocks for the theme, which isn't good.

   Note that we've seen this "all blocks get disabled" bug on rare occasions in Drupal Gardens and don't know exactly what causes it, but suspect this might be the root cause. (I also have a patch at #925360: Block module sometimes disables all blocks (root cause not yet found) to treat the symptom.)

3. Finally, there is really no fundamental reason why list_themes() should check that the theme data in the database is consistent with the filesystem. Code that needs to make sure that it is getting the latest data from the filesystem should already be calling (and does call) system_rebuild_theme_data() instead.

It looks like this file_exists() check has been around since at least Drupal 4.6, so it may be that it served a purpose a long time ago but no longer does.

The attached patch simply removes it.