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.