Remove code that was previously deprecated in 2.3.

• I've pushed back the removal of _app_ctx_stack and _request_ctx_stack until 2.4 to give extensions more time to update. The push and pop methods are removed, only top remains.
• FLASK_ENV env var, ENV config, and app.env
• session_cookie_name, send_file_max_age_default, use_x_sendfile, propagate_exceptions, and templates_auto_reload properties
• JSON_AS_ASCII, JSON_SORT_KEYS, JSONIFY_MIMETYPE, and JSONIFY_PRETTYPRINT_REGULAR config keys
• before_first_request
• json_encoder and json_decoder attributes, JSONEncoder and JSONDecoder classes
• htmlsafe_dumps and htmlsafe_dump