Deprecate and remove usages of datetime_date_default_time().

Problem/Motivation

The datetime module has one function it it that isn't a hook: datetime_date_default_time(). This function should not exist.

Proposed resolution

1. @deprecate it with the proper trigger_error().

2. Create a ::setDefaultDateTime() method on DateTimePlus that objects can use for date-only representations.

Remaining tasks

.

User interface changes

None.

API changes

datetime_date_default_time($date) is @deprecated

Use $date->setDefaultDateTime() instead.

Data model changes