Installer: Disable broken CACHE_DB after backend is disabled

Installer: Disable broken CACHE_DB after backend is disabled

We're currently not hitting any code that calls wfGetCache(), but
this seems like a good thing to do and would allow us to re-use
more code instead of duplicating a large method just because it
has a call to wfGetCache() somewhere.

This is already done for DB access and LocalisationCache in
general by the installer, but via SqlBagOStuff it can still
cause an exception from LBFactoryFake.

Change-Id: I69c1e34365c1d7503aea8ef47942d20c34318e85