Drop DC_P and DC_OAI datastreams

[giulioturetta]

Consider dropping DC_P and DC_OAI functional logic as the two data streams aren't useful anymore.

DC_P and DC_OAI data streams are created (or updated) every time update DC https://github.com/phaidra/phaidra-api/wiki/Documentation#update-dc is called whether or not updatedc hook is enabled or disabled in Phaidra API config.

Also, the two calls:

$r->route('object/:pid/dc') -> via('get') -> to('dc#get', dsid => 'DC_P');
$r->route('object/:pid/oai_dc') -> via('get') -> to('dc#get', dsid => 'DC_OAI');

do not make much sense when DC_P and DC_OAI data streams don't exist.