The relevant parts of the generated example documentation are already
`xi:include`d into the `migrating-gdbus.xml` page, so are turned into
HTML there. Installing them separately means they also get installed
into `/usr/share/gtk-doc/html/gdbus-object-manager-example/`, which
seems redundant.
Signed-off-by: Philip Withnall <withnall@endlessm.com>