Solrcas should remove CAS.toString() to field mapping aka "CasMapping"

Solrcas has a feature to map CAS.toString to a field in Solr. CAS.toString return the memory address of the CAS obejct, which in most contexts does not has any meaning to a user. This mapping should be removed since users might easily assume it could be used as an unique id which it cannot. There is also no reason to index the CAS memory address.

If a user wishes to index debugging information it should be added as FSes to the CAS and indexed. The functionality to do that is already present via type to field mappings.