remove ext_update from ext_autoload.php

adding the ext_update class inside dlf:ext_autoload.php breaks _all_ update
scripts of other extensions in TYPO3 > 6.0.

The error message points to the problem:

"class ext_update for this run does already exist, requiring impossible".

ext_update is available in plenty extensions. We must not preload it for
the dlf extension.
Merge pull request #15 from albig/master

remove ext_update from ext_autoload.php