Remove discrimination between URIs and literals of identifier

- it is ok if identifier are literals or if they are URIs. Both can be
  stored in the filesystem. This commit removes unnecessary log output
  which was noticed in project https://github.com/hbz/oerworldmap because
  there were URI's (urn's) used.
Merge pull request #404 from dr0i/RDFWriter-identifier-storing

Remove discrimination between URIs and literals of identifier