Reminder: officecfg/registry/schema/ is part of the stable API

Seeing various recent clean-up commits there, just a reminder that 
officecfg/registry/schema/ should be considered part of the stable 
3rd-party API (for LO extension etc.).

If you remove something from there:
* If an extension brings along .xcu data for such removed entities, that 
data should generally be silently ignored by configmgr (but removing 
e.g. a template completely might still cause exceptions to be thrown on 
start-up).
* But if an extension wants to access such a removed entity, it will get 
a (runtime) exception from configmgr (which will likely lead to a crash).

So please exercise the same care when doing incompatible changes to 
officecfg/registry/schema/ as when doing incompatible changes to other 
parts of the stable API.  Especially, it is not necessarily relevant how 
long a configuration entity has been unused by LO's code itself, if 
there may be 3rd-party code out there accessing that entity 
programmatically.