csync_update: Remove the check for csync_get_statedb_exists

csync_update: Remove the check for csync_get_statedb_exists

This always returns true since _csync_statedb_is_empty also always
returns true. This function was initially intended to be a shortcut
in case the database doesn't contain any row, but has been broken since
27fb5d9128c3d326191d5f9f01dac4f715000682 and now won't return false
unless the statement fails.

The _last_db_return_error in _csync_detect_update would take care
of that code path in a more direct and clearer way anyway.