Remove "seconds" field from cron expression

Current implementation uses a 6-field configuration for cron string so that we have a "seconds" parameters on the left, but this feature is not really needed and causes confusion because people tend to use https://crontab.guru/ for helping then define the cron string which is not really OK for current implementation.

Let's remove this field and certify that is is really compliant to Unix cron string expressions.

Locate the cron-expression configuration and parser, then compare the accepted format with Unix cron and crontab.guru examples. Confirm that the seconds field is removed and that representative five-field cron expressions are accepted.

Written by the indexing model from the issue text.