Add _IGNORE()s, remove obsolete comments

* gio/src/cancellable.hg: Remove obsolete comment.
* gio/src/dbusmessage.hg: Fix a typo. Remove obsolete comment. Add _IGNORE().
* gio/src/application.hg:
* gio/src/inputstream.hg:
* gio/src/outputstream.hg:
* gio/src/socketconnection.hg:
* glib/src/bytes.hg:
* glib/src/module.hg:
* glib/src/variant.hg: Add _IGNORE().