Fix EZP-21138: deleted nodes remain in an eZ Flow block

JIRA: https://jira.ez.no/browse/EZP-21138

# Description

The `ezflowcleanup.php` cronjob script checks for removed content as a result, it is not able to detect the removal of a location used in an eZ Flow block and thus the location remains in the block until the content is completely removed from eZ Publish. This patch makes the cronjob to check whether the location exists.

# Tests

manual tests