Remove eaccelerator config from conf.xml

This project is long since unsupported.

Existing conf.php files with eaccelerator configs will continue to work ...
we just don't provide an explicit configuration option anymore.
Bug #13264: Better documentation on good cache backend drivers
Keep data in session storage if using a Null cache driver with Horde_Core_Cache_Session

Bug #13264

It makes zero sense to run IMP (or any Horde app) with a null cache
driver, but I guess we need to support those broken configs.