MDL-18375 calendar: removed usage of the calendar type in the usergetdate function

The usergetdate function is not only used to display dates, it is also passed to the function make_timestamp.
This means users using another calendar type other than Gregorian will generate incorrect timestamps which may
be saved in the DB or used to populate the date_selector and date_time_selector elements. For example, when
creating an assignment using another calendar type other than Gregorian, the mod_form.php file calls
$this->apply_admin_defaults() which uses the function usergetmidnight to set the "Allow submissions from" date
to today's date and the "Due date" field to 7 days in the future. The usergetmidnight function calls usergetdate
which is then passed to make_timestamp. Since the usergetdate function was using the calendar type's
timestamp_to_date_array function the date being passed to make_timestamp was not in Gregorian. So, when using
the hijri calendar the year 1434 was being passed which was generating a large negative number as the timestamp
which was then used to populate the date fields.