Drop support for related objects and metadata in commit objects

Drop support for related objects and metadata in commit objects

While the actual commit object format is presently the same, for a
number of reasons we'd like to change it fairly radically.  Among
other things, we need to drop our a{sv} types in objects, to protect
against GVariant changing format.

Since now gnome-ostree now longer uses related objects, and nothing
ever used metadata, just drop them both.