This caused the syncroton Ping command to miss new task folders.
Because the Ping command is long running we use:
To clear the cache before listing the folders. However, this doesn't
clear the in-memory cache in kolab_storage.
In list_folders we then ended up in the branch returning the folder list directly from
that outdated in-memory cache and thus missed the update.
Since we already cache the folder list and metadata it doesn't seem necessary to
cache it again. Removing it thus gives us a single place to invalidate it.