remove meta:date-string element

See also OFFICE-1340 (I agree with the proposed resolution for ODF 1.2, but in ODF-Next I'd like to have it removed)

The <meta:date-string> element is only used in <office:annotation>, and I don't think a user will manually enter this date. If one really feels like it, one can still enter it as a separate <text>

(I assume that an office suite will set dc:date automatically, and displaying this date in a locale can be handled like any other date on other elements)