Fix unsetting of object properties

Properties on objects should never be cleared by using unset(), as this will not just set the property to NULL, but completely erase it, subsequently leading to notices like the following:

Notice: Undefined property: SearchApiSolrService::$solr in connect() (line 325 of …/search_api_solr/includes/service.inc).

We do this in SearchApiSolrService::setConnectionClass() and SearchApiSolrConnection::setBaseUrl(). The code still works fine, but the notices are still annoying and it should still be fixed.