dialogflow: remove unused samples + tests (#3249)

* dialogflow: delete unused samples and corresponding tests for context management * dialogflow: delete unused samples and corresponding tests for detecting intent with model selection * dialogflow: delete unused samples for document management * dialogflow: clean up document management whitespace * dialogflow: delete unused samples for entity management * dialogflow: delete tests for deleted entity management samples * dialogflow: delete tests for deleted document management samples * dialogflow: delete unused samples and corresponding tests for entity type management * dialogflow: delete unused samples and corresponding tests for session entity type management and entity type management * dialogflow: delete unused samples and corresponding tests for knowledge base management * flake8 lints Co-authored-by: Leah E. Cole <6719667+leahecole@users.noreply.github.com>

Author: Strykrol