Fix overzalous method annotations for drag and drop

Fix overzalous method annotations for drag and drop

Functional revert of commit 9c4892f2915.

Fixes introspection scanner warnings like:

  Warning: Gtk: gtk_drag_finish: Methods must belong to the same
  namespace as the class they belong to

That is, the gtk_drag_* functions cannot be methods as they have a
"GdkDragContext" as the instance parameter, and that is not a valid
type for the Gtk namespace.

This is not an introspected ABI change, as the generated introspection
data ignores the annotation.

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>
https://bugzilla.gnome.org/show_bug.cgi?id=692152