Enhancement: We improved the performance of db access

We removed a check for the existence of the db that was executed before every access to the db.

The check was introduced in #6049 to prevent crashes if the db does not exist or is removed during runtime.
We nowadays gracefully handle missing dbs on startup, removing the db at runtime is too much of a corner case
to sacrifice that much performance, however.

https://github.com/owncloud/client/pull/9918
