mtime of local folders is not set, so we should not check on it either.

As mentioned in https://github.com/owncloud/core/issues/7009#issuecomment-36616528 the modification time of a local folder is never touched. But in csync_update there is a check if the local mtime matches with the one from the database.

This commit doesn't trigger any action when the local modification time of a folder and the database differ. In the old scenario a CSYNC_INSTRUCTION_EVAL was triggered, resulting in a lot of additional useless propfinds for the folders.