[TASK] Remove custom "update" icon

With upcoming TYPO3 7 LTS, the special "download" icon does not look
good anymore. Instead of hacking the icon, just use the default one.

Change-Id: I43fb414075f87c7d09ad1372f51c1d7c2ca64086
Reviewed-on: http://review.typo3.org/33982
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>