[css3-gcpm] No need to call out attr() in bookmark-label and bookmark-target

In the definitions for 'bookmark-label' and 'bookmark-target', the
property definition line explicitly talks about the attr() function.
There's no need to do so - attr() resolves to a particular type based
on its type argument.  So bookmark-label, for example, only needs to
say that it accepts <string>, and then any use of "attr(foo as
string)" or "attr(foo)" will be valid, but "attr(foo as color)" won't
be.  A similar consideration applies for bookmark-target and the <url>
type.

~TJ