You can change all the AA prompts, but not that message. I timed it and you get 20 seconds of silence before the message. Instead of deleting the AA prompts you can replace them with a recording of a busy, dial tone or anything you want. The recording can be longer. The AA is single thread, so if a 2nd call arrives while the AA is processing a call, a real busy will be produced and the call will be routed to GV voicemail.
Use OBi Expert to make the following changes:
Voice Services -> Auto Attendant
DigitMap: (0)
OutboundCallRoute: {}
AnswerDelay: 0
Change all the Prompt values under Auto Attendant 1 Prompts to: &pause()
This will delete the prompt.
Are you selectively routing calls to AA or all calls?
Do you know how?