docs: remove unused dialogflow code samples (#563)

* remove dialogflow_delete_document region tag and code

* remove dialogflow_delete_knowledge_base  region tag and code

* remove dialogflow_detect_intent_with_model_selection region tag and code

* remove dialogflow_get_document region tag and code

* remove dialogflow_get_knowledge_base region tag and code

* remove dialogflow_list_document region tag and code

* remove dialogflow_list_knowledge_base region tag and code

* remove dialogflow_quickstart code

* delete all dialogflow_create* region tags and code

* remove dialogflow_delete* region tags and code

* remove dialogflow_list* region tags and code

* clean up comment span delimiters for deleted code

* Update detect.v2beta1.js

* Update detect.v2beta1.js

* Update resource.js

* Update detect.v2beta1.test.js

* Update resource.test.js

* Update detect.v2beta1.js

* docs: remove broken yargs commands from sample deletes

* Create create-document.v2beta1.test.js

* Create create-intent.test.js

* Create create-knowledge-base.v2beta1.test.js

* Create create-session-entity-type.test.js

* Create delete-intent.test.js

* Create delete-session-entity-type.test.js

* Update detect.v2beta1.test.js

* Create list-session-entity-types.test.js

* Update detect.v2beta1.js

* Delete resource.test.js

Co-authored-by: Your Name <you@example.com>
Co-authored-by: Noah Negrey <nnegrey@users.noreply.github.com>
Co-authored-by: Benjamin E. Coe <bencoe@google.com>