(examples revamp): remove redundant files

[tinalenguyen]

these examples will be removed because they are already present in another example or are already in the docs:

file	reason for removal
background_audio.py	docs
email_example.py	built in via GetEmailTask()
nvidia_test.py	functionally equivalent to basic_agent.py
realtime_video_agent.py	docs
realtime_with_ttst.py	docs