debian/authd.service: Don't create unused cache directory

We don't use the cache directory currently, and we still have code for
migrating the database from the old path (/var/cache/authd) to the new
path (/var/lib/authd), which results in this warning if both directories
exist:

    Both old and new database directories exist, can't migrate "/var/cache/authd/" to "/var/lib/authd/"

debian/authd.service.in

@@ -28,10 +28,6 @@ RestrictRealtime=yes
 RestrictSUIDSGID=yes
 SystemCallArchitectures=native
 
-# This always corresponds to /var/cache/authd
-CacheDirectory=authd
-CacheDirectoryMode=0700
-
 # This always corresponds to /var/lib/authd
 StateDirectory=authd
 StateDirectoryMode=0700