Do not use Reader::hrefToUid to determine the UID of an incidence

For some servers (e.g. Owncloud), the uid of an incidence cannot be derived from its uri. As a consequence, remote incidences are not correctly mapped to local incidences when Reader::hrefToUid is used.
This part is now handled by the if statement in line [452](https://github.com/nemomobile/buteo-sync-plugin-caldav/pull/34/files#diff-648c21f01a395ade305d1f1dc219c86bR452).