Remove the macros that used them, nobody could've used them anyway.
https://bugzilla.gnome.org/show_bug.cgi?id=687520
No need for per-instance checking of interface implementation here, presumably just a copy'n'paste issue.
The GstTagXmpWriter interface is to be implemented on elements that provide xmp serialization. It allows users to select which xmp schemas should be used on serialization. API: GstTagXmpWriter https://bugzilla.gnome.org/show_bug.cgi?id=645167
