registry: deprecate useless gst_registry_xml_{read|write}_cache()

registry: deprecate useless gst_registry_xml_{read|write}_cache()

The only reason these two functions are still around is that at some
point in the past they were in a public header, so we can't really
remove them now even though they should have been private all along
(and aren't really particularly useful). Since these are just empty
stubs now that do nothing but return FALSE and will be removed in
0.11 anyway, we may just as well deprecate them formally.