Remove segment warning when no start date is set before release #16200

See https://github.com/matomo-org/matomo/pull/16172/files#diff-c7ecf8d06820af52d6fae72297c6a9afR314

We should only trigger the warning if development mode is enabled.

Until the first few RCs we always show it so it's more visible. Alternatively, we could simply require a date to be always set and directly throw an exception in Segment::getSelectQuery() when no start or end date is set.

fyi @sgiehl