Always log cron execution

There was a setting to disable the last execution of cron. There is no known
problem with this write access and it was also questioned when this feature
was build in https://github.com/owncloud/core/pull/7689#issuecomment-38264707



Recently there was also a bug report about a non-visible last cron execution
(#6088) - let's better remove this.

Signed-off-by: Morris Jobke <hey@morrisjobke.de>