Drop useless initialization value

It is unused. ISO8601parseDate would either return false (and then
the function would throw), or it would use a different default and
result of parsing the string. The initializer here was confusing.

Change-Id: Ia44d0755262382fa94aefe0e8c183ba3785b5fe5
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/190773
Tested-by: Jenkins
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>

forms/source/xforms/convert.cxx

@@ -118,7 +118,7 @@ namespace
 
     css::util::Date lcl_toUNODate( std::u16string_view rString )
     {
-        css::util::Date aDate( 1, 1, 1900 );
+        css::util::Date aDate;
 
         bool bWellformed = ISO8601parseDate(rString, aDate);