(PUP-1586) remove obsolete unit test

The cron type unit test titled "it should require a command when adding
a new entry" did not quite do what it said. What it actually tested was
that the type would raise an exception when an empty command should-value
was accessed.

This can be safely removed, as PUP-1587 will implement a more sensible
protection against the creation of cronjobs that lack a command, including
a working integration test.