Revert the date cache feature as it somehow (cause: unknown) causing pageviews...

Revert the date cache feature as it somehow (cause: unknown) causing pageviews to be lost when importing big log files.
This particular log file I'm testing on is for an intranet with thousands times the same IP address. Not sure if it's related, but the same IP address will have many visits at the same second, for different users (different _id=X in the piwik.php requests)
refs https://github.com/piwik/piwik/pull/300