dialogflow: remove unused samples + tests [(#3249)](GoogleCloudPlatform/python-docs-samples#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