Make userdb cache as a built-in feature

Make userdb cache as a built-in feature

The disable-userdb-cache code path is never been fine tested and you
could expect an extramely slow bus if you did that. And there are known
bugs on fd.o if build without userdb cache. So to prevent user from
using bus without userdb cache, it changed to a built-in feature, no
longer optional now.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=66947
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=15589
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>