Remove QJSEngine::newDate() functions

Remove QJSEngine::newDate() functions

Rationale: Passing a QDateTime to toScriptValue() should be
sufficient, there's no need to have a dedicated factory
function for Date objects.

QDateTime has API for specifying a time in milliseconds, so the
newDate() overload that takes a double argument should be redundant.

Task-number: QTBUG-23604
Change-Id: Iec1d4ce8e1fd0a6ebef58dde3141730d27c2fb8c
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>